U.S. patent number 9,952,375 [Application Number 15/105,025] was granted by the patent office on 2018-04-24 for textured surfaces for display applications.
This patent grant is currently assigned to Corning Incorporated. The grantee listed for this patent is Corning Incorporated. Invention is credited to Daniel Aloysius Nolan, Vasudha Ravichandran, Ioannis Georgios Roudas, Christine Coulter Wolcott.
United States Patent |
9,952,375 |
Nolan , et al. |
April 24, 2018 |
Textured surfaces for display applications
Abstract
A substrate with a textured surface is disclosed. The substrate
may be, for example, a light emitter comprising a light guide, for
example a backlight element for use in a display device, wherein a
surface of the light guide, for example a glass substrate, is
configured to have a textured surface with a predetermined RMS
roughness and a predetermined correlation length of the texture. A
plurality of light scatter suppressing features can be provided on
the textured surface. Textured surfaces disclosed herein may be
effective to reduce electrostatic charging of the substrate
surface. Methods of producing the textured surface are also
disclosed.
Inventors: |
Nolan; Daniel Aloysius
(Corning, NY), Ravichandran; Vasudha (Painted Post, NY),
Roudas; Ioannis Georgios (Bozeman, MT), Wolcott; Christine
Coulter (Horseheads, NY) |
Applicant: |
Name |
City |
State |
Country |
Type |
Corning Incorporated |
Corning |
NY |
US |
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Assignee: |
Corning Incorporated (Corning,
NY)
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Family
ID: |
52302378 |
Appl.
No.: |
15/105,025 |
Filed: |
December 17, 2014 |
PCT
Filed: |
December 17, 2014 |
PCT No.: |
PCT/US2014/070771 |
371(c)(1),(2),(4) Date: |
June 16, 2016 |
PCT
Pub. No.: |
WO2015/095288 |
PCT
Pub. Date: |
June 25, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20160313494 A1 |
Oct 27, 2016 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61918276 |
Dec 19, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
6/0068 (20130101); G02B 5/0294 (20130101); C03C
17/32 (20130101); G02B 5/0278 (20130101); G02B
6/0036 (20130101); G02B 6/0061 (20130101); G02B
5/0221 (20130101); G02B 6/0065 (20130101); C03C
15/00 (20130101); C03C 2218/119 (20130101); G02B
1/118 (20130101) |
Current International
Class: |
F21V
7/04 (20060101); F21V 8/00 (20060101); G02B
5/02 (20060101); C03C 15/00 (20060101); G02B
1/118 (20150101) |
Field of
Search: |
;362/613 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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101191860 |
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Jun 2008 |
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CN |
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03215218 |
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Oct 2001 |
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JP |
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2014041284 |
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Mar 2014 |
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JP |
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2014049243 |
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Mar 2014 |
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JP |
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2009103731 |
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Oct 2009 |
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KR |
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99/33910 |
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Jul 1999 |
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WO |
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2012058084 |
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May 2012 |
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WO |
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Other References
Barboux et al.; "Paradoxical crystalline morphology of frosted
glass"; Journal of Non-Crystalline Solids 345&346 (2004); pp.
137-141. cited by applicant .
Barrett et al.; "The Determination of Pore Volume and Area
Distributions in Porous Substances. I. Computations from Nitrogen
Isotherms"; J. Am. Chem. Soc., 1951, 73 (1); pp. 373-380. cited by
applicant .
European Patent Office; International Search Report and Written
Opinion for International Application No. PCT/US2014/070771; dated
Jul. 20, 2015; pp. 1-15. cited by applicant .
Sahraei et al.; "Optimum feature size of randomly textured glass
substrates for maximum scattering inside thin-film silicon solar
cells"; Proc. of SPIE, vol. 8981 (2014); pp. 89811D-1-89811D-8.
cited by applicant .
Kim et al.; "Analysis of thermo-physical and optical properties of
a diffuser using PET/PC/PBT copolymer in LCD backlight units";
Displays 26 (2005); pp. 37-43. cited by applicant .
Lin et al.; "Flatness and microstructure of a light guide plate
fabricated by microinjection molding"; Polymer Engineering and
Science, vol. 53, Issue 1 (Jan. 2013); pp. 212-218. cited by
applicant .
Beckmann et al.; "The Scattering of Electromagnetic Waves from
Rough Surfaces"; Pergamon Press Ltd. 1963; pp. 3-97. cited by
applicant.
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Primary Examiner: Kim; Ahshik
Attorney, Agent or Firm: Able; Kevin M.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of priority under 35 U.S.C.
.sctn. 371 of International Patent Application Serial No.
PCT/US14/70771, filed Dec. 17, 2014, which claims the benefit of
priority under 35 U.S.C. .sctn. 119 of U.S. Provisional Application
Ser. No. 61/918,276 filed on Dec. 19, 2013, the contents of which
are relied upon and incorporated herein by reference their
entireties.
Claims
What is claimed is:
1. A substrate comprising at least one textured major surface
comprising an RMS roughness R.sub.q where 5 nm.ltoreq.Rq.ltoreq.75
nm, the texture of the textured major surface including a
correlation length T where 0 nm<T.ltoreq.150 nm, the substrate
further comprising a haze value equal to or less than 6.0% and a
transmittance normal to the at least one major surface greater than
90% over a wavelength range from 400 nm to 700 nm.
2. The substrate according to claim 1, wherein the substrate
comprises a glass layer.
3. The substrate according to claim 2, wherein the substrate
comprises a polymer layer disposed on the glass layer.
4. The substrate according to claim 3, wherein the textured major
surface is a surface of the polymer layer.
5. The substrate according to claim 1, wherein the substrate
comprises a first substrate and further comprising a second
substrate joined to the first substrate to form a substrate
assembly such that the textured surface is positioned within an
interior of the substrate assembly.
6. The substrate according to claim 1, wherein the substrate is a
chemically strengthened substrate.
7. The substrate according to claim 1, wherein the substrate is a
laminated substrate comprising a first glass layer including a
first coefficient of thermal expansion and a second glass layer
fused to the first glass layer and comprising a second coefficient
of thermal expansion different from the first coefficient of
thermal expansion.
8. The substrate according to claim 3, wherein the polymer layer
comprises a plurality of discrete light scatter suppressing
features, the discrete light scatter suppressing features
comprising an index of refraction substantially equal to an index
of refraction of the glass layer.
9. The substrate according to claim 8, wherein a spatial density of
the plurality of light scatter suppressing features varies as a
function of distance from an edge of the substrate.
10. The substrate according to claim 4, wherein the polymer layer
is a continuous polymer layer.
11. The substrate according to claim 10, wherein the continuous
polymer layer is a polymer film applied to the glass layer.
12. The substrate according to claim 5, wherein the second
substrate is a chemically strengthened substrate.
13. The substrate according to claim 5, wherein the second
substrate is a laminated substrate comprising a first layer
including a first coefficient of thermal expansion and a second
layer comprising a second coefficient of thermal expansion
different from the first coefficient of thermal expansion.
14. The substrate according to claim 13, wherein the first and
second layers comprise glass.
15. The substrate according to claim 1, wherein the substrate
comprises a scatter ratio defined as a diffuse transmittance
divided by a total transmittance equal to or greater than 0.5.
16. The substrate according to claim 1, wherein the substrate
comprises an optical attenuation equal to or less than 26 dB/meter
in a wavelength range from 400 nm to 700 nm.
17. The substrate according to claim 1, wherein the substrate
comprises a light guide.
18. The substrate according to claim 1, further comprising a light
source positioned adjacent an edge surface of the substrate.
19. The substrate according to claim 18, wherein the light source
comprises a plurality of light sources.
20. The substrate according to claim 1, wherein the substrate
further comprises a frame disposed about a perimeter of the
substrate.
21. A light guide comprising a substrate including at least one
textured major surface comprising an RMS roughness R.sub.q where 5
nm.ltoreq.Rq.ltoreq.75 nm, the texture of the textured major
surface including a correlation length T where 0 nm<T.ltoreq.150
nm, the light guide further comprising a haze value equal to or
less than 6.0% and a transmittance normal to the at least one major
surface greater than 90% over a wavelength range from 400 nm to 700
nm.
22. The light guide according to claim 21, wherein 5
nm.ltoreq.R.sub.q.ltoreq.40 nm.
23. The light guide according to claim 22, wherein 0
nm<T.ltoreq.100 nm.
24. The light guide according to claim 21, wherein a transmittance
of the light guide normal to the at least one major surface is
equal to or greater than 94%.
25. The light guide according to claim 21, wherein the light guide
has a haze value equal to or less than 6.0%.
26. The light guide according to claim 21, further comprising a
plurality of light scatter suppressing features deposited on the
textured major surface.
27. The light guide according to claim 26, wherein a spatial
density of the plurality of light scatter suppressing features
varies as a function of distance from an edge of the substrate.
28. The light guide according to claim 21, wherein the substrate is
a glass substrate.
Description
BACKGROUND
Field
The present invention relates generally to textured surfaces for
display applications. For example, a textured surface as disclosed
herein may be used in a light emitter (e.g., a backlight for
certain display devices), the light emitter comprising a substrate
with a textured surface on one or both major surfaces of the
substrate that can operate as a light guide, and processes for
making the light emitter. The light guide may include a pattern of
discrete dots of material deposited thereon to control the light
output of the light emitter. Textured surfaces disclosed herein may
also be used as projections surfaces for viewing images from a
projecting image source. In certain embodiments micro-replication
(nano-replication) techniques may be used to efficiently and
economically produce the textured surface. Textured surfaces
disclosed herein may even be used to mitigate electrostatic
charging for display glass substrates during certain manufacturing
processes.
Technical Background
Conventional components used to produce diffused light have
included diffusive structures, including polymer light guides and
diffusive films which have been employed in a number of
applications in the display industry. These applications include
bezel-free television systems, liquid crystal displays (LCDs),
electrophoretic displays (EPD), organic light emitting diode
displays (OLEDs), plasma display panels (PDPs),
micro-electromechanical structures (MEMS) displays, electronic
reader (e-reader) devices, and others.
The desire for thinner, lighter and more energy efficient displays
have led to the development of so-called transparent displays. Long
a staple of science fiction, transparent displays are now being
commercially implemented in several variations, including vending
machine doors, freezer doors, retail advertising, augmented reality
screens, heads-up displays in the automotive industry, smart
windows for offices, portable consumer electronics, and security
monitoring.
Unfortunately, transparent displays are susceptible to several poor
performance characteristics. In actuality, currently available
displays only partially transmit and reflect light, thus the
contrast ratio of the display is greatly limited. Commercially
available transparent displays typically offer only about 15%
transmission, and performance is even lower in reflection mode.
For many practical applications, a transparent display requires the
support of backplane illumination (via a transparent back light
element). To maintain transparency, the back light needs to be
fully transparent in an OFF-state and fully illuminated in an
ON-state. Back lights having a frosted appearance are generally
unacceptable. Additionally, the use of a transparent back light
necessarily eliminates the use of a conventional reflective medium.
Existing technology for providing backplane illumination are not
satisfactorily meeting certain cost and performance requirements of
the marketplace for transparent displays.
Other components of display devices, particularly the substrates
forming the display panel itself, may present problems during
manufacture of the thin film devices formed on at least one of the
substrates. For example, triboelectrification can impede the
ability to remove the substrate from surfaces in contact with the
substrate, which can lead to breakage of the substrate and particle
generation in some cases. This can be particularly troublesome for
substrates, such as glass substrates, produced via processes that
result in extremely smooth surfaces on the substrate.
SUMMARY
Conventional transparent display systems, such as transparent and
semi-transparent LCD televisions, are commercially available for
digital signage and advertising applications. These display systems
are semi-transparent in the OFF-state (i.e., when no image is being
commanded by the associated electronics driving the LCD elements).
To maintain the semi-transparent characteristics, these
conventional display systems do not employ an opaque optical light
emitter to produce light. Instead, these display systems use
background ambient light to illuminate the LCDs in the ON-state
(i.e., when the associated electronics is commanding an image), a
so-called "white box" approach. In the white box approach, a large
"box", such as a food vending machine, includes one or more
reflective interior surfaces, for example surfaces coated with
white paint, and the box is brightly lighted from within. The
window to the box, for example a door, comprises a transparent
display panel, wherein the brightly lighted box serves as a light
source for the display window. Thus, one can see through the
display and view objects (such as merchandise, etc.) behind the
display panel. Concurrently, the viewer can also receive visual
information on certain portions or the entirety of the display
panel, which in a commercial application might be related to the
merchandise behind the display panel.
As discussed above, a significant issue with current "transparent"
display systems is that they are not particularly transparent. In
fact, measurements have shown they exhibit only about a 15%
transmission ratio. Thus, a relatively high level of ambient light
may be needed in proximity to the display panel, which might not
always be feasible or even desirable for any number of reasons. The
result is an array of problems related to uniformity of image
quality across the display, such as non-uniformity in color,
contrast ratio, etc.
Thus, in accordance with one or more embodiments presented herein,
a light emitter is described. As used herein, the term light
emitter refers generically to a device configured to provide
illumination. The light emitter may, for example, include a back
light element for use in a display device, or the light emitter may
be configured to provide general illumination such as illumination
for a room or vehicle. The light emitter includes a light guide
into which light can be coupled by one or more slight sources and
through which the coupled light propagates. The light guide is
generally a substrate comprising opposing major surfaces. As used
herein, the term substrate refers generally to a plate-like
substrate and which in some embodiments is suitable for use as a
light guide. The substrate may be planar, or the substrate may, in
some examples, have a wedge-like shape. Nevertheless, the substrate
generally has two opposing major surfaces bounded by edge surfaces.
The major surfaces may be planar and parallel, or the major
surfaces may be planar and non-parallel. In some embodiments the
substrate may be curved, for example as used in a curved display
device. For light emitter applications at least one of the opposing
major surfaces includes a surface texture configured to scatter at
least a portion of the light propagating within the light guide.
The surface texture is specifically configured to render the light
guide visually transparent, without appreciable haze, thereby
making the light guide particularly useful in the construction of a
back light element for use in a transparent display device. The
configuration of the surface texture also provides excellent
viewing angle performance.
The light emitter, for example a back light element comprising a
light guide, may be positioned behind a transparent display panel
relative to a viewer of the display panel. Light may be coupled
into the light guide along one or more edge surfaces of the
substrate, and/or along one or more borders thereof, wherein the
borders represent portions of the major surfaces proximate the edge
surfaces. The light propagates in a waveguide fashion within the
light guide, for example by total internal reflection, and is
incident on the light scattering portion of the at least one major
surface. Thus, light propagating through the light guide and which
light may be incident on a textured surface of the substrate may be
scattered out of the light guide to illuminate the display panel of
the display device, such as an LCD display panel. The light emitter
described herein can function as an improved light source for the
LCD elements of a transparent display system over currently
available alternatives by eliminating the need for complex
scattering and turning films, which decrease the light output from
conventional back light elements. In addition, the surface texture
produces a level of haze low enough to achieve a high level of back
light element transparency when portions of the display system are
in the OFF-state. In examples, the haze may be equal to or less
than 6%, equal to or less than 4%, equal to or less than 2% or
equal to or less than 1%, it being understood that haze is bounded
at its minimum by 0%. In most cases, a haze equal to or less than
1% is desired. As used herein, the term transparent, or variations
thereof, is intended to convey visual transparency (for example
over a wavelength range from about 400 nm to about 700 nm), wherein
an object placed behind the transparent element can be distinctly
seen by an observer from the opposite side of the transparent
element, the transparent element being disposed on the sight line
between the observer and the object.
In use as a back light element, the light emitter can provide
additional lighting output over conventional back light elements to
increase the brightness, functionality and the viewing angle of a
transparent display system. In other uses, the scattering structure
of the light guide and its use in a light emitter can be employed
in a variety of applications as a source of light, including but
not limited to architectural lighting, automotive lighting,
decorative lighting and the like. Accordingly, the light emitter in
various aspects described herein is not limited to a display back
light element, but may be used in a variety of different
applications where a source of light may be needed, the foregoing
list naming but a few. Moreover, display applications may include
devices extending beyond televisions, computer displays, laptop,
tablet and phone displays and can include, for example, sky lights,
virtual reality goggles or other wearable display devices.
In the context of a display device, the addition of a visually
transparent back light element can eliminate the need for a
reflective medium in the transparent display, which eliminates a
major disadvantage of reflective-type displays). The transparent
display may therefore work solely in transmission, eliminating
image distortion from glare and reflection. Another advantage to
eliminating the reflective medium is in providing an improved color
balance. By working in transmission-only mode, the transparent
display device can have an improved image rendering quality.
Still further, the light emitters described herein can provide
improvements in color fidelity by enabling the tuning of light
coupled from the light source into the light guide. When such
tuning includes selecting colors to mix with the natural color
shade of the transparent display device, the color fidelity of the
display device is improved. Additionally, in one or more further
embodiments, the light coupled into the light guide may be adaptive
(by tuning the light sources, e.g., controlling LEDs of different
colors) to enhance scene colors based on instantaneous color
content.
The primary advantage of an etching approach according to the
present disclosure for making textured surfaces, such as textured
surfaces for display applications, and particularly when the
substrate is glass, is that it can be accomplished with only wet
solutions using chemicals commonly available in industrial
quantities. The use of commonly available chemicals can
significantly lower the cost of the process compared with other
techniques, such as abrasive etch, etching particles, and/or
aqueous etching cream processes. The solutions required for the
etching techniques disclosed herein may also not include a common
but very hazardous hydrofluoric acid component, which results in a
much safer etch mixture for production both in terms of worker
safety and environmental quality. The process can produce a narrow
distribution of lateral feature sizes when compared with some other
processes.
In architectural and aesthetic applications, light emitters as
described herein may be employed for both indoor and outdoor
lighting, both functional and decorative. For example, the light
emitter may be used as a skylight, where the textured glass would
be relatively unaffected by weather conditions. The advantages of
using a substantially transparent light emitter as a skylight
include high resistance to discoloration from weathering, the
ability to provide artificial light (not simply channel sunlight)
in low lighting conditions, such as at night or on cloudy days to
provide additional lighting. The transparency of a light emitter
employed as a skylight can allow viewing the overhead sky during
daylight hours, yet provides illumination to the underlying space
during low light or evening hours. Moreover, an advantage of light
emitters described in certain examples herein is the ability to
provide uniform light output across a surface thereof, if so
desired, by the application of light scatter suppressing features
on the textured surface of the light guide. As the name implies,
such light scattering features function to suppress or completely
eliminate scattering of incident light at the feature location. In
some embodiments, such as decorative applications, light output
from the light emitter may be intentionally configured to be
non-uniform, either by eliminating the light scatter suppressing
features, or by the intentional placement of a pattern of the light
scatter suppressing features so as to produce a desired effect. The
range of such designed patterns is limited only by the designer's
imagination.
In yet another application, certain transparent substrates can be
utilized in a projection mode, wherein a projection system projects
an image onto a visually transparent substrate such that the
transparent substrate functions as a projection screen. In such
applications the projected image is readily viewable on the
transparent substrate, and yet the substrate is simultaneously
substantially transparent so that the background behind the screen
is clearly visible. Ordinarily, such an image projected onto a
transparent substrate, for example a glass plate not configured to
scatter light, will not be readily visible at the transparent
substrate. Instead, light rays from the projection system will be
substantially transmitted through the substrate making it difficult
if not impossible to view the projected image on the substrate.
Textured substrates as described herein, however, are capable of
providing sufficient scattering of the incident light to make the
image visible, while maintaining the substrate essentially
transparent to the viewer without discernible haze. Touch
capability can be added to the projection screen by adding
touch-functional layers to the projection screen in a manner as is
known in the art. Such touch functionality can be accomplished by
using transparent oxide materials that do not significantly affect
the transparency of the projection screen.
In still other applications, substrates utilizing textures
disclosed herein can be positioned in front of but spaced apart
from an image device, for example a large screen television device
or a computer monitor, as an overlay. Again, touch capability can
be incorporated with the substrate which can be electronically
coordinated with the displayed image, thereby adding touch
capability to the image device that was not previously present. The
substrate can be entirely structurally disconnected from the image
device, although electrical communication may be required.
Accordingly, a controller can be used to relate the touch (contact)
location on the overlay panel with a particular location on the
display panel image being displayed immediately behind that touch
location, and then initiate a predetermined action.
Accordingly, the light emitter for these and other uses, and more
particularly the light guide comprising the light emitter, can, in
addition to being transparent (e.g. exhibit minimal haze), be
configured to be suitably rigid and may, according to some
examples, have a bulk modulus in a range equal to or greater than
10 gigaPascals (GPa). For example, the bulk modulus can range from
about 10 GPa to about 445 GPa. For example, in a range from about
30 GPa to about 250 GPa, or in a range from about 30 GPa to about
100 GPa. Glass, for example, has a bulk modulus in a range from
about 35 to about 55 GPa. In contrast, sapphire has a bulk modulus
of about 250 GPa.
To ensure suitable transmittance within the interior of the light
guide, the light guide may have an optical loss equal to or less
than 26 dB per meter in a wavelength range from about 400
nanometers to about 700 nanometers. In some examples the optical
loss can be equal to or less than 17 dB per meter, whereas in
certain other examples the optical loss can be equal to or less
than 10 dB/meter. Suitable materials include without limitation
glass materials including silica-based glasses, crystalline
materials such as sapphire or diamond and diamond-like materials,
or other materials, including calcium fluoride, as the quality of
light emitted from the light guide is more a function of the
surface characteristics rather than the bulk material.
A scatter ratio of the light guide, defined as the diffuse
transmittance divided by the total transmittance, may be equal to
or greater than 0.5. Moreover, for applications needing uniform
illumination, a brightness variation of the light guide across an
emitting surface of the light guide can be equal to or less than
20%. A viewing angle goodness parameter (VAG), described herein
below, may be equal to or greater than 1.0.
Still further, particular processes for producing glass light
guides described herein are compatible with both pre and
post-process ion-exchange treatments. The use of chemically
strengthened glass through ion exchange may be used to provide
protection for other delicate structures, such as the LCD panel in
a display device.
Compared to other techniques, illumination using light emitters
described herein can be very uniform, light transmission is high,
and, in the case of glass, the size of the substrate that can be
prepared is not particularly limited.
Accordingly, disclosed herein is a substrate comprising at least
one textured major surface comprising an RMS roughness R.sub.q
where 5 nanometers.ltoreq.R.sub.q.ltoreq.75 nanometers, for example
in a range from about 5 nanometers to about 40 nanometers. The
texture of the textured major surface may also include a
correlation length T where 0 nanometers<T.ltoreq.150 nanometers,
for example in a range from greater than zero to equal to or less
than 100 nanometers. The substrate may further comprise a haze
value equal to or less than 6.0% and a transmittance normal to the
at least one major surface greater than 90% over a wavelength range
from 400 nm to 700 nm. The substrate may comprise a glass layer.
The substrate may comprise a polymer layer. For example, the
substrate may comprise a glass-polymer laminate including a polymer
layer disposed on the glass layer.
In some embodiments the textured major surface is a surface of a
polymer layer.
In certain examples the substrate can comprise a first substrate
and may further comprise a second substrate joined to the first
substrate to form a substrate assembly with the textured surface
positioned within an interior of the substrate assembly. For
example, the textured surface can be a surface of a polymer
material disposed on the first substrate, and a second substrate is
positioned such that the polymer layer including the textured
surface is sandwiched between the first and second substrates. Both
substrates may include a glass layer. For example, in some
embodiments the first substrate may be a glass plate and the second
substrate may be a glass plate. The polymer layer may be, for
example, a pre-textured film produced in a replication process, for
example a micro- or nano-replication process, and positioned
between the first and second glass plate.
The substrate may be a chemically strengthened substrate, for
example an ion-exchanged glass substrate. In some embodiments the
substrate may be a laminated substrate comprising a first layer
including a first coefficient of thermal expansion and a second
layer comprising a second coefficient of thermal expansion
different from the first coefficient of thermal expansion. For
example, the laminate may be a glass laminate wherein two layers of
glass having two different compositions and two different
coefficients of thermal expansion may be fused together to produce
the laminate. The glass laminate can be produced, for example, via
a fusion process wherein multiple streams of molten glass of
different compositions are flowed over the surfaces of forming
bodies and joined to produce a single glass stream that cools and
can be cut into individual glass sheets. The differing coefficients
of thermal expansion can produce a compressive stress in the
surfaces of the glass sheets.
The polymer layer according to some embodiments may comprise a
plurality of discrete light scatter suppressing features, the
discrete light scatter suppressing features comprising an index of
refraction substantially equal to an index of refraction of the
glass layer. For example, the index of refraction of the light
scatter suppressing features can be within .+-.10% of the index of
refraction of the substrate. A spatial density of the plurality of
light scatter suppressing features can be arranged to vary as a
function of distance from an edge of the substrate to produce a
predetermined light output as a function of position on a surface
of the substrate. For example, a spatial density of the plurality
of light scatter suppressing features can be arranged to produce a
substantially uniform light output for use as a back light for a
display device.
In other embodiments the polymer layer may be a continuous layer,
such as a polymer film applied to a glass layer, e.g. the first
glass substrate. The second substrate may be a chemically
strengthened substrate. For example, in embodiments comprising two
glass substrates, both glass substrates can be chemically
strengthened substrates. Alternatively, the second substrate is a
laminated substrate comprising a first layer including a first
coefficient of thermal expansion and a second layer comprising a
second coefficient of thermal expansion different from the first
coefficient of thermal expansion. Both the first and second
substrates can be laminated substrates comprising a plurality of
glass compositions with different coefficients of thermal
expansion.
Preferably, the substrate comprises a bulk modulus M, where 10
gigaPascals.ltoreq.M.ltoreq.450 gigaPascals. The substrate
preferably comprises a scatter ratio defined as a diffuse
transmittance divided by a total transmittance equal to or greater
than 0.5. The substrate preferably comprises an optical attenuation
equal to or less than 26 dB/meter in a wavelength range from about
400 nm to about 700 nm.
When used as a light emitter, for example a light guide in a back
light element, the substrate may further comprise a light source,
or a plurality of light sources, positioned adjacent an edge
surface of the substrate.
In some embodiments the substrate may further comprise a frame
disposed about a perimeter of the substrate. For example, the
substrate may be configured as a projection screen or a display
overlay.
In another embodiment a light emitter is disclosed comprising a
light guiding substrate comprising a first edge and a second edge
opposite the first edge, the substrate further comprising at least
one textured surface with an RMS roughness in a range from about 5
nanometers to about 75 nanometers, for example in a range from
about 5 nanometers to about 40 nanometers. A correlation length of
the texture may be in a range from greater than zero nanometers to
about 150 nanometers, for example in a range from about 5
nanometers to about 100 nanometers. The substrate may, for example,
be a glass substrate, and more particularly the glass substrate may
be a silica-based glass substrate. The substrate may include a
plurality of discrete deposited features on the textured surface
that function as light scatter suppressing features. A spatial
density of the deposited features may vary as a function of
distance in a direction from the first edge to the second edge. For
example, the spatial density of the deposited features may decrease
as a function of distance in a direction from the first edge to the
second edge. In one example, the spatial density of the deposited
features may vary from about 95% to about 5% in a direction from
the first edge to the second edge, the spatial density determined
as a percent coverage of a unit area. The spatial density of the
deposited features may be substantially constant along a line
parallel with the first edge. The deposited features may be locally
randomly distributed.
Preferably, the deposited features are transparent. The deposited
features may comprise, for example, a polymer resin. An index of
refraction of the deposited features may be close to or match a
refractive index of the substrate. For example, an index of
refraction of the deposited features may be within 10% of the index
of refraction of the substrate. Typically, a refractive index of
the substrate is in a range from about 25.times.10.sup.-7/.degree.
C. to about 300.times.10.sup.-7/.degree. C. over a temperature
range from 25.degree. C. to 300.degree. C. The light emitter may be
transparent such that in an OFF state the light emitter can be
viewed through, and in the ON state the light emitter emits
light.
As will become clear from the following description, the light
emitter when configured as a back light element may also function
as an opaque light emitter by employing a reflective panel at the
back side of the light guide farthest from the display panel.
Accordingly, the light emitter disclosed herein may be used where
transparency is not required and a greater light output is
needed.
The substrate may include a surface porosity characterized by a
bimodal parameter (BP) value in a range from about 0.16 to about
0.22.
In another aspect, a display device is disclosed comprising: a
display panel, a light emitter, for example a back light element
positioned adjacent to the display panel, the back light element
comprising a light guide including a substrate with a first edge
and a second edge opposite the first edge. The substrate further
comprises at least one textured surface with an RMS roughness in a
range from about 5 nanometers to about 75 nanometers, for example
in a range from about 5 nanometers to about 40 nanometers. A
correlation length of the surface texture may be in a range from
greater than zero nanometers to about 150 nanometers, for example
in a range from 5 nanometers to about 100 nanometers The textured
surface may include a plurality of discrete deposited features
deposited thereon to control light scattering from the light guide.
The deposited features serve as light scatter suppressing features.
For example, the deposited features may be used to selectively
control the scattering of light from the textured surface of the
substrate. Accordingly, the deposited features may be configured to
produce a uniform illumination from the light emitter. Thus, a
spatial density of the deposited features can decrease as a
function of distance from the first edge to the second edge. In
other examples the deposited features may be arranged to produce an
intentional pattern of illumination, for example a non-uniform
pattern such as for decorative lighting. In display applications
the textured surface of the light guide may be positioned to face
away from the display panel. In other examples the textured surface
can be arranged to face toward the display panel. In still other
examples, both major surfaces of the light guide may be textured.
For use in applications where transparency is not a benefit, the
light emitter may be opaque and incorporate, for example a
reflective element positioned adjacent a major surface of the light
guide to increase light output of the light emitter.
To ensure proper control of light scattering by the light guide,
and therefore light extraction, the deposited features may comprise
an index of refraction close to or matched with the index of
refraction of the substrate. For example, the index of refraction
of the deposited features may be within 10% of the index of
refraction of the underlying light guide substrate and be
considered refractive index-matched. The deposited features may,
for example, be a polymer resin. In other examples the deposited
features may include transparent metal oxide layers such as a
transparent conductive oxide (TCO) used in the manufacture of
semiconductor devices.
The spatial density of the deposited features can vary from about
95% to about 5% in a direction from the first edge to the second
edge, for example in a range from about 75% to about 25%, the
spatial density being determined as a percent coverage of a unit
area. Thus, the spatial density may be a gradient from one edge in
a direction toward another edge, or even from one edge to a middle
of the glass substrate between two parallel edges. As used herein,
an edge may be distinguished from an edge surface in that an edge
represents the boundary between an edge surface and a major surface
of the substrate. For example, although in some embodiments
illustrated in the accompanying drawings the light guide is shown
with light coupled from the light source(s) through a single edge
surface of the substrate, in other embodiments not shown, light may
be coupled from two or more edge surfaces of the substrate.
Suitable light sources include light emitting diodes (LEDs). In
some examples, light may be coupled from a border area of a major
surface proximate an edge (perimeter) of the major surface.
Accordingly, the pattern of the deposited features, and therefore
the spatial density thereof, may be adjusted to obtain the desired
light output uniformity. However, on a local scale, the deposited
features may be randomly distributed.
In still another embodiment a method of forming a light guide is
described comprising treating a surface of a glass substrate with
an etchant comprising acetic acid, for example glacial acetic acid,
in an amount from about 92% to about 98% by weight, ammonium
fluoride in an amount from about 0.5% to about 5.5% by weight, and
less than 6% by weight water. While not wishing to be bound by any
particular theory, it is believed that for some embodiments the
lower the water content, the lower the ammonium fluoride
dissociation rate, which leads to a reduced release of HF. A
reduced concentration of HF can also lead to a reduction in the
removal of glass from the glass substrate, which in turn results in
shallower pits in the etched surface thereof, which is needed for
low haze (e.g. greater transparency). Additionally, reduced water
content can provide for a reduction in the solubility of the
precipitated crystal mask in the etchant. This means the mask stays
on the glass substrate surface longer. When the nucleated crystals
stay undissolved on the glass surface, the etch rate for the
etchant is lower and the crystal lateral dimension remains
unchanged, thus leading to the desired low correlation length.
The method may further comprise depositing a plurality of dots
(deposited features) of a transparent material on the textured
surface of the glass substrate. The deposited features serve as
light scatter suppressing features. For example, the deposited
features may be used to selectively control the scattering of light
from the textured surface of the substrate. The dots comprising the
deposited material may have an index of refraction that is close to
or matches a refractive index of the glass substrate, for example
within .+-.10%. A spatial density of the deposited features
preferably decreases as a function of distance from a first edge of
the glass substrate to a second edge of the glass substrate
parallel with the first edge to produce a graduated spatial
density. The deposited dots can, for example, comprise a polymer
resin. The graduated spatial density may comprise a linear
graduation or a non-linear graduation.
The spatial density of the deposited features can vary from about
95% to about 5% in a direction from a first edge of the glass
substrate toward a second edge of the glass substrate, for example
in a range from about 75% to about 25%, the spatial density being
determined as a percent coverage of a unit area. The spatial
density of the deposited features (dots) can be substantially
constant along a line parallel with the first edge.
In some embodiments the light guide may include a suitable
underlying substrate comprising a polymer layer deposited thereon.
The underlying substrate provides rigidity to the polymer layer,
whereas the polymer layer provides the necessary surface roughness
characteristics. The underlying substrate may include any of the
foregoing materials, including glass. The polymer layer may, in
some examples, have a thickness less than 100 micrometers. A
surface texture may then be produced in the exposed surface of the
polymer layer such as by pressing (stamping) with a master mold
(e.g. an etched glass plate produced by the methods disclosed
herein). In some embodiments the polymer layer may be a film and
the texture may be produced on the polymer layer prior to its
application to the glass layer.
In aspects where a glass substrate is required to minimize
electrostatic charging, the etchant may be adjusted. For example,
in some embodiments the etchant comprises acetic acid
(HC.sub.2H.sub.3O.sub.2) in a range from about 10% by weight to
about 90% by weight, for example in a range from about 10% to about
80% by weight, in a range from about 20% to about 70% by weight, in
a range from about 20% by weight to about 30% by weight or in a
range from about 40% by weight to about 60% by weight. The etchant
may further include NH.sub.4F in a range from about 1% to about 50%
by weight, for example in a range from about 5% to about 40% by
weight, in a range from about 10% by weight to about 30% by weight.
In certain embodiments polyethylene glycol or other organic
solvents can be substituted for the acetic acid. Depending on the
desired texture, etching times can be in a range from about thirty
seconds to two minutes, and in some cases up to four minutes. When
the etching is complete, the glass substrate can be soaked in 1M
H.sub.2SO.sub.4 for up to 1 minute to remove precipitated crystal
residue on the surface of the glass substrate. However, in some
embodiments other mineral acids may be substituted for
H.sub.2SO.sub.4, for example HCl or HNO.sub.3. In some cases hot
water may be sufficient. A low pH value and/or a high temperature
can increase the solubility of the crystals. Finally, the glass
substrate may be rinsed with water (e.g. deionized water) and
dried. To avoid increased production costs associated with high
temperature etching, the etching process can be performed at room
temperature, with the understanding that higher temperature
etching, if desired, falls within the scope of the method.
Additional features and advantages of the embodiments described
herein will be set forth in the detailed description which follows,
and in part will be readily apparent to those skilled in the art
from that description or recognized by practicing the embodiments
as described herein, including the detailed description which
follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description
and the following detailed description present embodiments of the
present disclosure, and are intended to provide an overview or
framework for understanding the nature and character of the
embodiments as they are claimed. The accompanying drawings are
included to provide a further understanding of the embodiments, and
are incorporated into and constitute a part of this specification.
The drawings illustrate various embodiments of the present
disclosure and together with the description serve to explain the
principles and operations of the embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional side view of a display device according
to an embodiment described herein;
FIG. 2 is a top down view of the display device according to FIG.
1, showing a distribution of dots of a deposited material on a
roughened surface;
FIG. 3 is a cross sectional side view of a lighting apparatus
(backlight) comprising the display device of FIG. 1;
FIG. 4 is a plot showing modeled data of normalized power versus
scattering angle for a textured surface having an RMS roughness of
50 nm and several correlation lengths;
FIG. 5 is a cross sectional side view of another embodiment of a
backlight wherein the glass substrate comprising the backlight
includes at least one angled edge;
FIG. 6 is a cross sectional side view of another display device
wherein the lighting apparatus comprises a fiber optic light
source;
FIG. 7 is a cross sectional side view of a lighting apparatus
comprising the display device of FIG. 6 wherein different
wavelengths of light can be coupled into the fiber optic light
source;
FIG. 8 depicts a process flow diagram for a method of producing a
lighting apparatus according to an embodiment of the present
disclosure;
FIG. 9 is a cross sectional side view of a glass substrate for use
in a backlight as disclosed herein, wherein a protective laminate
is applied to a surface of the glass substrate;
FIG. 10 is a schematic diagram of an example nano-replication
process that can be used to produce a polymer film with a textured
surface;
FIG. 11 is a drum used to produce a textured surface in the process
shown in FIG. 10;
FIG. 12 is a perspective drawing of a projection system for
projecting an image onto a transparent projection screen;
FIGS. 13A, 13B and 13C are edge views of several embodiments of
projection screens useable with the projection system of FIG.
12;
FIG. 14 is a plot of luminance versus viewing angle for an example
backlight etched under different etching conditions as described
herein;
FIG. 15 is a plot showing luminance as a function of viewing angle
for 12 different backlights;
FIG. 16 is a plot illustrating the correlation between the
correlation length of the glass substrate surface texture and the
viewing angle goodness parameter;
FIG. 17 is a plot illustrating the correlation between a unit-less
porosity bi-modality parameter characterizing the glass substrate
surface texture and the viewing angle goodness parameter;
FIG. 18 is a photomicrograph of a single dot deposited on a
textured surface of a glass substrate (deposited feature);
FIG. 19 is another photomicrograph of another dot deposited on a
textured surface of a glass substrate (deposited feature);
FIG. 20 is a plot showing the dimensions of the deposited feature
of FIG. 14;
FIG. 21 is a plot showing measured brightness as power scattered
through a surface of the glass substrate in a region comprising a
surface texture without dots, and a region comprising texture and
dots;
FIG. 22 is a plot showing the relationship between glass substrate
temperature during embodiments of the etching process described
herein and the viewing angle goodness parameter on two separate
days;
FIGS. 23A, 23B and 23C are photomicrographs of a glass substrate
surface showing deposited crystalline precipitates resulting from
an etching process on the glass substrate surface;
FIG. 24 is a plot showing the effect of a conventional etching
process on several different glasses;
FIG. 25 is a plot illustrating surface voltage reduction for glass
samples etched by different chemistries as a function of average
roughness R.sub.a;
FIG. 26A is a plot comparing surface roughness as a function of
etch time for glass samples wherein an acetic acid/ammonium
fluoride etchant was static and wherein the etchant was
agitated;
FIG. 26B is a plot comparing surface roughness as a function of
etch time for glass samples wherein an polyethylene glycol/ammonium
bifluoride etchant was static and wherein the etchant was
agitated;
FIG. 27A depicts haze as a function of R.sub.a obtained through in
situ mask etching approach with a solution of 20% by weight
NH.sub.4F and 50% by weight acetic acid;
FIG. 27B plots haze as a function of R.sub.a for glass etched in a
solution of 11% by weight NH.sub.4FHF and 25% by weight
polyethylene glycol;
FIG. 28A through FIG. 28F are AFM plots of the surfaces for 6
samples C0 through C5, respectively, wherein C0 is a control sample
and C1 through C5 are samples etched with various etchants and
showing the surface texture of each surface.
FIG. 29 is a plot showing peak surface voltage resulting from the
application of a peel test for four different etchants to samples
C1-C5, and a control sample C0;
FIGS. 30A and 30B are a plots showing the degree of correlation
between FOM (m=2, 6) and electrostatic charge-induced glass surface
voltage for various topographical embodiments C0-C5;
FIGS. 31A and 31B are contour plots showing the effect of FOM
calculation parameters on the correlation coefficient R.sup.2, i.e.
the ability to best correlate with electrostatic charge voltage
response;
FIGS. 32A and 32B illustrate an idealized surface topographies and
a profile thereof (in arbitrary units), respectively, that assist
in describing the unique space wherein FOM and Ra (R.sub.q)
diverge;
FIGS. 33A and 33B illustrate another idealized surface topographies
and a profile thereof (in arbitrary units), respectively, that
assist in describing the unique space wherein FOM and Ra (R.sub.q)
diverge;
FIGS. 34A and 34B illustrate still another idealized surface
topographies and a profile thereof (in arbitrary units),
respectively, that assist in describing the unique space wherein
FOM and Ra (R.sub.q) diverge;
FIG. 35 is a graphical illustration of FOM vs. duty cycle for the
idealized surface topographies of FIGS. 32A through 34B, the
surface having a constant roughness R.sub.a (or R.sub.q) and
showing the relative dependence of FOM on surface area and
independence from roughness;
FIG. 36 is a graphical illustration of FOM as a function of
roughness (R.sub.a) showing the independence of FOM from roughness;
and
FIG. 37 is a schematic illustration showing electrostatic charge
response as a function of FOM and highlighting examples where
roughness (R.sub.a or R.sub.q) is insufficient and/or doesn't
uniquely describe contact-area/electrostatic charge reduction.
DETAILED DESCRIPTION
Conventional back light elements for illuminating visual display
devices are typically made using polymers such as polymethyl
methacrylate (PMMA) or polycarbonate. Such materials are relatively
inexpensive and offer good light transmission. As display devices
become slimmer, however, the light guides comprising the back light
must also become thinner. For example, in some handheld devices
these light guides are less than 1 millimeter thick. Thin polymer
light guides become expensive to fabricate. In addition, very thin
polymer light guides degrade quickly under constant exposure to the
light and heat emitted from the LED sources during device use (e.g.
photo-aging), especially near the light coupling zone where the
LEDs are arrayed. Additionally, manufacturing thin warp-free
polymer sheets is challenging, and exposure to a non-uniform
temperature field during display operation can cause polymer light
guides to warp, leading to a defect called "pooling mura". Polymers
also have a low thermal conductivity, approximately 0.18 watts per
meter per Kelvin (W/mK) for PMMA, and 0.19 W/mK for polycarbonate.
Consequently, polymer light guides have a reduced ability to
dissipate heat buildup near the light coupling zone and can
discolor and become brittle. Moreover, polymer light guides
typically also display poor angular uniformity of luminance, such
that the peak intensity of light emitted from a light guide
produced from materials like PMMA and polycarbonate can occur at
angles approaching approximately 70 degrees as measured from a
normal to the light guide surface. Consequently, optical films are
often utilized at the front surface of these light guides to
produce scattering and turn the light in the normal direction.
Unfortunately, such optical films typically have poor transparency,
making them unsuitable for use in a transparent display. As used
herein, the front surface refers to the surface from which the
light is emitted from the light guide for lighting purposes. For a
liquid crystal display, the front surface of the light guide, as
used herein, is the surface facing the LCD panel. Accordingly, the
back side is the surface facing away from the display panel.
For at least the reasons above, alternatives that offer greater
rigidity, less warping, less discoloration and exhibit an optical
loss sufficiently low to produce viable light emitters are
attractive alternatives. Material options include without
limitation glass, sapphire, certain diamond-like materials or even
calcium fluoride.
Glass, in particular, is not susceptible to photo-aging and
discoloration under the prolonged light exposure conditions typical
of a display device. Moreover, glass, particularly silica-based
glass, can be produced with very high thickness uniformity in
exceptionally transparent sheets at low cost compared to other
materials that might otherwise be suitable. As a result, glass can
be an ideal light guide in a light emitter, and is particularly
well suited for applications in a back light element for display
devices. Accordingly, the following description and examples
present methods and articles based on glass, with the understanding
that other materials may be used in the manufacture of a textured
substrate having the characteristics, e.g. surface texture,
disclosed herein.
In accordance with one or more embodiments disclosed herein, a
"maskless" etching technique may be used to produce glass light
guides that can be used in any number of applications, including as
a light guide for a backlight element in transparent or
non-transparent display applications. The methodology involves a
low cost, highly controllable process for producing a light
scattering portion that includes nanometer-sized features
distributed in and/or on a surface of a glass substrate. In
addition, micrometer-sized light scatter suppressing features may
be deposited over the nanometer-sized features on the glass
surface. The combination of etched nanometer-sized features and
deposited micron-sized light scatter suppressing features
tremendously improves the light emitting properties of the light
guide. In operation, light is coupled into and propagates through
the glass substrate in a waveguide fashion and is incident on the
light scattering surface portion. In response, the light scatters
out of the glass structure across a surface thereof in accordance
with desirable optical characteristics. However, where the light
propagating through the glass substrate encounters a light scatter
suppressing feature, total internal reflection may be restored, and
the light continues propagating through the glass substrate without
being scattered out. Thus, light extraction from the glass
substrate via scattering is suppressed, and light output may
thereby be controlled by controlling the distribution of light
scatter suppressing features (dots) on the surface of the glass
substrate.
FIG. 1 shows a cross-sectional view of a textured light guide
substrate as a dielectric slab waveguide illuminated from an edge
thereof by an array of light emitting diodes (LEDs). In the ray
optics illustration of FIG. 1, light rays emanate from the LEDs and
propagate in the dielectric slab waveguide in a zigzag pattern,
each ray reflecting alternatively from the two major surfaces of
the glass substrate. If the interfaces of the slab waveguide with
the surrounding environment are perfectly smooth and the angle that
each ray forms with an interface at which the ray is incident is
greater than a critical angle .phi..sub.e, relative to a normal to
that interface, the ray bounces off the interface at the same angle
as the incident angle and stays confined within the waveguide. This
phenomenon is called total internal reflection.
However, total internal reflection breaks down when texture
features at an interface of the light guide at the point of
incidence are small compared to the wavelength of the incident
light. That is, when the surface of the light guide at the point of
incidence is roughened and the features comprising the texture are
very small some light scattering occurs at each bounce of the light
ray on the textured interface of the light guide. Scattering
produces light rays at angles different than the incident angle.
Some of these rays do not satisfy the total internal reflection
criterion and escape from the textured interface of the slab
waveguide and can illuminate the LCD panel at various angles. Some
rays may continue to reflect at or greater than the critical
angle.
The methodologies described herein involve a wet chemical etching
process for texturing a glass substrate for illumination
applications and articles made by that process that may be
incorporated into subsequent devices, such as the aforementioned
display applications. Additionally, such treatments may be applied
to glass substrates for other purposes, such as suppressing
electrostatic charging of substrates during processing of such
substrates.
Creating texture on glass surfaces using fluoride-containing
solutions requires an etch mask, since without a mask amorphous
homogenous silicate glass tends to etch evenly on a scale larger
than the molecular level, reducing a thickness of the glass, but
without creating texture. Many methods have been proposed for
masking glass etching to provide patterned textures for various
applications. Such methods can be divided into those requiring a
separate masking process prior to etching and those which form a
mask in situ during etching, so-called "maskless" etching since
there is no mask prior to the start of the etching. For the
purposes of this disclosure, a mask can be considered any material
that provides a barrier to etching, and may be applied to a glass
surface with various lateral feature sizes and with various levels
of durability and adhesion to the glass.
Many methods of mask application, such as ink jet printing, have
limitations on the scale of the mask that can be applied in that
they do not enable the deposition of small, nanometer-sized
features. In fact, most methods produce textures that are on a
micrometer scale in both lateral feature size and depth of etch and
so create a visible "frosted" appearance to glass that reduces
transparency, increases haze, and decreases glare and surface
reflectivity.
In situ masking and glass etching involves a complex process of
mask formation from byproducts of glass dissolution plus etchant.
Precipitates that form (sometimes crystalline) are often somewhat
soluble in the etchant, making modeling of this process difficult.
Moreover, creating a differential etch using maskless etching may
involve multiple steps to create the mask by contact with a
frosting solution or gel, and subsequent steps to remove the mask
and etchant. In situ etch masks can also produce various textures
depending on their adhesion to the substrate and durability in the
wet etchant, and it can be shown that less durable masks result in
shallower textures. Depth of etch is also determined by the size of
the mask features, with smaller features unable to support deeper
etch profiles because mask undercutting occurs more readily.
Therefore, mask chemistry, glass chemistry, and etch chemistry
should all be considered when forming nanometer-scale textures.
Disclosed herein is a glass substrate with a textured surface
suitable as a light guide plate for a light emitter with a broad
angular distribution of luminance, and a process for making the
same utilizing a modified maskless etch method. The method produces
texture on a glass surface that is useful for a display light guide
applications and any other applications where controlled
illumination is desired, and in particular a transparent source of
light. Methods disclosed herein may also be used to produce
textured surfaces that reduce electrostatic charging without a
noticeable formation of haze.
The present disclosure describes substrates and light guides that
enhance thermal stability, have a relatively low coefficient of
thermal expansion and resistance to thermal breakdown, can produce
uniform luminance over a broad viewing angle range with high
brightness, and exhibit transparency with low haze. The surface
texture properties that have been observed and can be responsible
for these desirable optical properties may include a correlation
length of the texture features equal to or less than 150 nm and a
root mean square (RMS) roughness (R.sub.q) of the textured surface
in a range from about 5 nm to about 75 nanometers, for example in a
range from about 5 nanometers to about 40 nm, in a range from about
5 nanometers to about 30 nm, and in a range from about 5 nanometers
to about 20 nm. Generally, the smaller the value of the RMS
roughness, the better the performance of the light guide in terms
of its scattering characteristics, as the influence of roughness on
physical quantities can depend, inter alia, on the correlation
properties and/or spatial frequencies present on the surface. The
notion of a correlation length is introduced, the correlation
length being related to the autocorrelation function
G.sub.x(.tau..sub.x) of a surface expressed as a one-dimensional
function z(x), where
.function..tau.>.infin..times..times..times..intg..times..function..ti-
mes..function..tau..times..times..times. ##EQU00001## where L
represents one half the profile length of the sample scan (distance
over which the autocorrelation function is determined), z is the
height of the surface relative to a mean height, x represents a
position on the surface in the x direction, .tau. is the distance
between two positions x.sub.1 and x.sub.2. Sampling of the surface
obtains an array of heights z(j) across M positions spaced .DELTA.x
apart. Here, k is an integer from 0 to M-1. Approximating for
.tau..sub.x=k.DELTA..sub.x, equation (1) can be reduced to equation
(2) below,
.times..times..times. ##EQU00002##
The autocorrelation function describes how the surface is
correlated to itself at a distance .tau..sub.x. The autocorrelation
function goes to zero for large .tau..sub.x such that the heights
of the surface features become independent. Thus, one can refer to
the autocorrelation length T, or simply the correlation length to
represent the characteristic lateral feature size of a surface. To
obtain the correlation length from measured data, one may select,
for example, a Gaussian model for G.sub.x(.tau..sub.x), for example
of the form,
.function..tau..sigma..times..tau. ##EQU00003## calculate a
discretized G.sub.k from measured data (using, for example, a fast
Fourier transform), and fit the model on the discretized
autocorrelation function. Alternative approaches may use an
exponential model. Calculations used herein employed a Gaussian
model and all correlations lengths are expressed as a Gaussian
correlation length unless otherwise stated. In other embodiments
the calculations above may be altered to include a two-dimensional
consideration of the surface rather than a one dimensional
approach. Software programs such as Gwyddion, an open-source
software platform developed and supported by the Department of
Nanometrology, Czech Metrology Institute intended for analysis of
height fields obtained by scanning probe microscopy techniques to
calculate an autocorrelation function are available that can
compute the correlation length based on measured data.
These chemical etch methods can be divided into three areas needed
to make specific textures resulting in good light guide optical
properties: 1) etchant chemistry, 2) glass chemistry, and 3)
process steps.
As described supra, the chemical etch process includes etching the
glass substrate in an etchant, for example an etchant bath, that
comprises a mixture of an organic solvent such as acetic acid (AA)
and an inorganic acid such as ammonium fluoride (NH.sub.4F). The
glass structure resulting from exposure to the etchant is a
textured glass substrate. For illumination applications, the
textured glass substrate may be optimized by controlling parameters
of the process, such as but not limited to the composition of the
etchants, the etching time, the etchant temperature and the glass
temperature. A reliance on the addition of alkali or alkaline earth
salts for removal of the mask is unnecessary.
Other additives may provide advantages as well. These may include:
A dye to add color to the etchant and enable a visual aid for
rinsing (common food-grade dye can suffice), and viscosity
modification components to thicken the acid and enable painting or
spraying the etchant on glass as opposed to dipping. A thickened
acid can also prove advantageous since this is likely to reduce the
vapor pressure and thereby reduce defects caused by acid vapor
contact with the substrate. For example, a suitable thickening
agent is polycaprolactone, a synthetic polyester which melts at
60.degree. C. and which can be used to make a wax etchant.
Polycaprolactone is soluble in acetic acid and insoluble in
water.
The process for modifying a glass substrate with an etched texture
on at least one surface thereof may in some embodiments comprise at
least six steps, all of which can be performed within a temperature
range from about 18.degree. C. to about 22.degree. C. (it should be
noted that acetic acid begins to freeze at temperatures below
approximately 17.degree. C. Accordingly, a minimum temperature
should be greater than 17.degree. C.).
In a first step of an example process, the glass substrate to be
etched is cleaned using a detergent to remove all inorganic
contamination, then rinsed sufficiently to remove detergent
residue. A level of cleanliness sufficient to obtain a water
contact angle of less than about 20.degree. C. should be attained.
Contact angle can be evaluated using, for example, a DSA100 drop
shape analyzer manufactured by Kruss GmbH and employing a sessile
drop method, which was done for experiments described herein,
although other suitable methods may also be used. In an optional
second step of the example process the back side of the glass
substrate can be laminated with a suitable adhesive polymer film
(or other acid barrier), if only a single side of the glass
substrate is to be etched. The polymer film can be removed from the
glass substrate after the etching process is completed. It should
be noted that a similar process can be used if the opposite (back
side) of the substrate is to be etched, wherein the polymer film is
added to the front surface of the substrate.
In a third step of the example process, the glass substrate is
contacted with an etchant for a time sufficient to create the
desired texture (typically in a range from about 0.5 minutes to 6
minutes). For immersion, fast insertion and suitable environmental
controls, for example an ambient air flow of at least a 2.83 cubic
meter per minute in the enclosure in which the etching occurs may
be used to limit exposure of the glass substrate to acid vapor
prior to and/or during insertion. The glass substrate should be
inserted into the acid bath using a smooth motion to prevent
defects forming in the etched surface. The glass substrate should
be dry prior to contact with the etchant.
In a fourth step, the glass substrate is removed from the etchant
and allowed to drain, then rinsed one or more times with water, for
example deionized water, or alternatively with a solution in which
the precipitant is dissolvable. Agitation of either the glass
substrate or the rinse liquid sufficient to ensure uniform
diffusion of fluoride-containing acid clinging to the glass
substrate may be performed. Rinse steps can employ agitation to
prevent defects. Small oscillations of approximately 300
oscillations per minute are sufficient, for example between about
250 and 350 oscillations per minute. The etchant bath may in some
instances be recirculated to prevent stratifications and
depletion.
In a fifth step of the example process any etchant-blocking film
previously applied to the back side of the glass substrate can be
removed, such as by peeling.
In a sixth step of the example process, the glass substrate can be
dried using forced clean (filtered) air to prevent water spots, or
spots from other rinsing solutions, from forming on the glass
substrate.
The example process outlined briefly above is capable of providing
the specific textures and optical properties described herein, and
also to enable high uniformity of etch for each sample when
combined with features from the following detailed description.
With reference to FIG. 1, not shown to scale, an edge view of a
display device 10 is depicted comprising a display panel 12, such
as an LCD display panel, and a light emitter 14 in accordance with
one or more embodiments described herein. Light emitter 14 may be
employed to process light for a display system or other
application.
In reference to FIGS. 1 and 2, light emitter 14 comprises a light
source 16 and a glass substrate 18 that serves as a light guide.
Light coupled into glass substrate 18 from light source 16
propagates within the glass substrate and is scattered at a major
surface of glass substrate 18 and directed, in the present figure,
through an opposite surface of glass substrate 18, as indicated by
arrows 20. Glass substrate 18 comprises first and second spaced
apart major surfaces 22, 24. First and second major surfaces 22 and
24 may in some examples be parallel with one another. Glass
substrate 18 may have a rectangular shape with a first edge 26 and
a second edge 28 opposite to and parallel with first edge 26. First
and second major surfaces 22, 24 extend between first and second
edges 26, 28.
For purposes of description and not limitation, the distance
between first and second edges 26, 28, as best seen with the aid of
FIG. 2, will be referred to as the length L of glass substrate 18.
Accordingly, glass substrate 18 further comprises a width W
extending between third and fourth edges 30 and 32, wherein third
edge 30 is parallel with fourth edge 32, and both third and fourth
edges 30, 32 are generally perpendicular to and intersect with
first and second edges 26 and 28.
Glass substrate 18 may be any suitable glass that can withstand the
processing parameters expressly or inherently disclosed herein may
be employed as glass substrate 18, for example an alkali silicate
glass, an aluminosilicate glass, or an aluminoborosilicate glass.
The glass material may be a silica-based glass, for example code
2318 glass, code 2319 glass, code 2320 glass, Eagle XG.RTM. glass,
Lotus.TM., and soda-lime glass, etc., all available from Corning,
Inc. Other display-type glasses may also benefit from the processes
described herein. Thus, glass substrate 18 is not limited to the
previously described Corning Incorporated glasses. For example, one
selection factor for the glass may be whether a subsequent ion
exchange process may be performed, in which case it is generally
desirable that the glass be an alkali-containing glass.
Display glass substrates can have various compositions and be
formed by different processes. Suitable formation processes
include, but are not limited to float processes and downdraw
processes such as slot-draw and fusion draw processes. See, for
example, U.S. Pat. Nos. 3,338,696 and 3,682,609. In the slot-draw
and fusion draw processes, the newly-formed glass sheet is oriented
in a vertical direction. One glass substrate, Lotus.TM.,
manufactured by Corning, Inc., has a small coefficient of thermal
expansion and is superior in dimensional stability and workability
at relatively high processing temperatures. Lotus.TM. glass
contains little, if any, alkali components in the glass.
Suitable glass display substrates include high performance glass
substrates manufactured by Corning, Inc. The glass substrates are
specifically designed to be used in the manufacture of flat panel
displays and exhibit densities of less than 2.45 g/cm.sup.3 and a
liquidus viscosity (defined as the viscosity of the glass at the
liquidus temperature) greater than about 200,000 poises, or greater
than about 400,000 poises, or greater than about 600,000 poises, or
greater than about 800,000 poises. Additionally, suitable glass
substrates exhibit substantially linear coefficients of thermal
expansion over the temperature range of 0.degree. to 300.degree. C.
of 28-35.times.10.sup.-7/.degree. C., or of
28-33.times.10.sup.-7/.degree. C., and strain points higher than
about 650.degree. C. As used in this specification and the appended
claims, the term "substantially linear" means that the linear
regression of data points across the specified range has a
coefficient of determination greater than or equal to about 0.9, or
greater than or equal to about 0.95, or greater than or equal to
about 0.98 or greater than or equal to about 0.99, or greater than
or equal to about 0.995. Suitable glass substrates include those
with a melting temperature less than 1700.degree. C. In addition,
suitable glass substrates may exhibit a weight loss of less than
0.5 mg/cm.sup.2 after immersion in a solution of 1 part HF (50 wt.
%) and 10 parts NH.sub.4F (40 wt. %) for 5 minutes at 30.degree.
C.
In one embodiment of the described process, the glass substrate has
a composition in which the major components of the glass are
SiO.sub.2, Al.sub.2O.sub.3, B.sub.2O.sub.3, and at least two
alkaline earth oxides. Suitable alkaline earth oxides include, but
are not limited to MgO, BaO and CaO. The SiO.sub.2 serves as the
basic glass former of the glass and has a concentration greater
than or equal to about 64 mole percent in order to provide the
glass with a density and chemical durability suitable for a flat
panel display glass, e.g., a glass suitable for use in an active
matrix liquid crystal display panel (AMLCD), and a liquidus
temperature (liquidus viscosity) which allows the glass to be
formed by a downdraw process (e.g., a fusion process) described in
more detail below. Suitable glass substrates have a density less
than or equal to about 2.45 grams/cm.sup.3, or less than or equal
to about 2.41 grams/cm.sup.3, a weight loss which is less than or
equal to about 0.8 milligrams/cm.sup.2 when a polished sample is
exposed to a 5% HCl solution for 24 hours at 95.degree. C., and a
weight loss of less than 1.5 milligrams/cm.sup.2 when exposed to a
solution of 1 volume of 50% by weight HF and 10 volumes 40% by
weight NH.sub.4F at 30.degree. C. for 5 minutes.
Suitable glass for use with embodiments of the present disclosure
have an SiO.sub.2 concentration less than or equal to about 71 mole
percent to allow batch materials to be melted using conventional,
high volume melting techniques, e.g., Joule melting in a refractory
melter. In detailed embodiments, the SiO.sub.2 concentration is in
a range from about 66.0 to about 70.5 mole percent, or in a range
from about 66.5 to about 70.0 mole percent, or in a range from
about 67.0 to about 69.5 mole percent. As a result of the SiO.sub.2
content, suitable glasses may have melting temperatures equal to or
greater than about 1600.degree. C.
Aluminum oxide (Al.sub.2O.sub.3) is another glass former suitable
for use with embodiments of the disclosure. Without being bound by
any particular theory of operation, it is believed that an
Al.sub.2O.sub.3 concentration equal to or greater than about 9.0
mole percent provides a glass with a low liquidus temperature and a
corresponding high liquidus viscosity. The use of at least about
9.0 mole percent Al.sub.2O.sub.3 may also improve the strain point
and the modulus of the glass. In detailed embodiments, the
Al.sub.2O.sub.3 concentration may be in the range from about 9.5 to
about 11.5 mole percent.
Boron oxide (B.sub.2O.sub.3) is both a glass former and a flux that
aids melting and lowers the melting temperature. To achieve these
effects, glasses for use with embodiments of the present disclosure
can have B.sub.2O.sub.3 concentrations equal to or greater than
about 7.0 mole percent. Large amounts of B.sub.2O.sub.3, however,
lead to reductions in strain point (approximately 10.degree. C. for
each mole percent increase in B.sub.2O.sub.3 above 7.0 mole
percent), modulus, and chemical durability.
Suitable glass substrates may have a strain point equal to or
greater than about 650.degree. C., equal to or greater than about
655.degree. C., or equal to or greater than about 660.degree. C., a
Young's modulus equal to or greater than 10.0.times.10.sup.6 psi,
and a chemical durability as described above in connection with the
discussion of the SiO.sub.2 content of the glass. Without being
bound by any particular theory of operation, it is believed that a
high strain point may help prevent panel distortion due to
compaction/shrinkage during thermal processing subsequent to
manufacturing of the glass. Accordingly, it is believed that a high
Young's modulus may reduce the amount of sag exhibited by large
glass sheets during shipping and handling.
In addition to the glass formers (SiO.sub.2, Al.sub.2O.sub.3, and
B.sub.2O.sub.3), suitable glass substrates may also include at
least two alkaline earth oxides, i.e., at least MgO and CaO, and,
optionally, SrO and/or BaO. Without being bound by any particular
theory of operation, it is believed that alkaline earth oxides
provide the glass with various properties important to melting,
fining, forming, and ultimate use. In some embodiments, the MgO
concentration is greater than or equal to about 1.0 mole percent.
In detailed embodiments, the MgO concentration is in the range of
about 1.6 and about 2.4 mole percent.
Of the alkaline earth oxides, the CaO concentration of some
embodiments of the glass substrate is the largest. Without being
bound by any particular theory of operation, it is believed that
CaO produces low liquidus temperatures (high liquidus viscosities),
high strain points and moduli, and coefficients of thermal
expansion (CTE's) in the most desired ranges for flat panel
applications, specifically, AMLCD applications. It is also believed
that CaO contributes favorably to chemical durability, and compared
to other alkaline earth oxides, CaO is relatively inexpensive as a
batch material. Accordingly, in some embodiments, the CaO
concentration is greater than or equal to about 6.0 mole percent.
In specific embodiments, the CaO concentration in the display glass
is less than or equal to about 11.5 mole percent, or in the range
of about 6.5 and about 10.5 mole percent.
In some examples, the glass may comprise 60-70 mol % SiO.sub.2;
6-14 mol % Al.sub.2O.sub.3; 0-15 mol % B.sub.2O.sub.3; 0-15 mol %
Li.sub.2O; 0-20 mol % Na.sub.2O; 0-10 mol % K.sub.2O; 0-8 mol %
MgO; 0-10 mol % CaO; 0-5 mol % ZrO.sub.2; 0-1 mol % SnO.sub.2; 0-1
mol % CeO.sub.2; less than 50 ppm As.sub.2O.sub.3; and less than 50
ppm Sb.sub.2O.sub.3; wherein 12 mol
%.ltoreq.Li.sub.2O+Na.sub.2O+K.sub.2O.ltoreq.20 mol % and 0 mol
%.ltoreq.MgO+CaO.ltoreq.10 mol %, and wherein the silicate glass is
substantially free of lithium.
Certain glass substrates described herein can be what is referred
to in the art as laminated glass. In one aspect, the display glass
substrate is produced by fusion drawing a glass skin to at least
one exposed surface of a glass core. Generally, the glass skin will
possess a strain point equal to or greater than 650.degree. C. In
some embodiments, the skin glass composition has a strain point
equal to or greater than 670.degree. C., equal to or greater than
690.degree. C., equal to or greater than 710.degree. C., equal to
or greater than 730.degree. C., equal to or greater than
750.degree. C., equal to or greater than 770.degree. C., or equal
to or greater than 790.degree. C. The strain point of the disclosed
compositions can be determined by one of ordinary skill in the art
using known techniques. For example, the strain point can be
determined using ASTM method C336.
In some embodiments, the glass skin can be applied to an exposed
surface of a glass core by a fusion process. An example of a
suitable fusion process is disclosed in U.S. Pat. No. 4,214,886,
which is incorporated by reference herein in its entirety. The
fusion glass substrate forming process can be summarized as
follows. At least two glasses of different compositions (e.g., the
base or core glass sheet and the skin) are separately melted. Each
of the glasses is then delivered through an appropriate delivery
system to a respective overflow distributor. The distributors are
mounted one above the other so that the glass from each flows over
top edge portions of the distributor and down at least one side to
form a uniform flow layer of appropriate thickness on one or both
sides of the distributor. The molten glass overflowing the lower
distributor flows downwardly along the distributor walls and forms
an initial glass flow layer adjacent to the converging outer
surfaces of the bottom distributer. Likewise, molten glass
overflowing from the upper distributor flows downwardly over the
upper distributor walls and flows over an outer surface of the
initial glass flow layer. The two individual layers of glass from
the two distributers are brought together and fused at the draw
line formed where the converging surfaces of the lower distributor
meet to form a single continuously laminated ribbon of glass. The
central glass in a two-glass laminate is called the core glass,
whereas the glass positioned on the external surface of the core
glass is called the skin glass. A skin glass can be positioned on
each surface of the core glass, or there may be only one skin glass
layer positioned on a single side of the core glass. When just one
skin glass is fused directly to the core, the skin is "adjacent" to
the core.
The overflow distributor process provides a fire polished surface
to the glass ribbon so formed, and the uniformly distributed
thickness of the glass ribbon provided by the controlled
distributor(s), and the glass sheets cut therefrom, provides the
glass sheets with superior optical quality. The glass sheets used
as display glass substrates can have a thickness in the range of
100 micrometers (.mu.m) to about 0.7 .mu.m, but other glass sheets
that may benefit from the methods described herein may have a
thickness in a range from about 10 .mu.m to about 5 mm. Other
fusion processes, which can be used in the methods disclosed
herein, are described in U.S. Pat. Nos. 3,338,696, 3,682,609,
4,102,664, 4,880,453, and U.S. Published Application No.
2005/0001201, which are incorporated by reference herein in their
entireties. The fusion manufacturing process offers advantages for
the display industry, including glass that is flat with excellent
thickness control and glass that has a pristine surface quality and
scalability. Glass substrate flatness can be important in the
production of panels for liquid crystal display (LCD) televisions
as any deviations from flatness can result in visual
distortions.
In some embodiments, the glass substrate will possess a strain
point equal to or greater than 640.degree. C., a coefficient of
thermal expansion in a range from about 31.times.10.sup.-7/.degree.
C. to about 57.times.10.sup.-7/.degree. C., a weight loss less than
20 mg/cm.sup.2 after immersion for 24 hours in an aqueous 5% by
weight HCl solution at about 95.degree. C., that is nominally free
from alkali metal oxides and has a composition, calculated in
weight percent on the oxide basis, comprising about 49 to 67%
SiO.sub.2, at least about 6% Al.sub.2O.sub.3,
SiO.sub.2+Al.sub.2O.sub.3>68%, B.sub.2O.sub.3 in a range from
about 0% to about 15%, at least one alkaline earth metal oxide
selected from the group consisting of, in the preparations
indicated, about 0 to 21% BaO, about 0 to 15% SrO, about 0 to 18%
CaO, about 0 to 8% MgO and about 12 to 30% BaO+CaO+SrO+MgO.
In accordance with embodiments disclosed herein and shown in a
close up view in FIG. 3, at least one surface of glass substrate
18, e.g. first major surface 22, is treated to include a plurality
of nanometer-scale light scattering elements 34. As will be shown
below, the plurality of light scattering elements 34 produce a
texture on first major surface 22 and the texture can be formed by
a modification of first major surface 22 through the action of
etching with an etchant as described supra and disclosed in greater
detail below.
A light ray 36 entering glass substrate 18 through first edge 26
propagates through the glass substrate under conditions of total
internal reflection in a direction generally along axis 38 until
the ray of light impinges on scattering elements 34. For example,
if the angle .phi. between a normal 40 to a surface of the glass
substrate is greater than the critical angle .phi..sub.c, where
.phi..sub.c can be calculated according to equation (4) below,
.phi..sub.c=arc sin(n.sub.2/n.sub.1) (4) where n.sub.1 is the index
of refraction of glass substrate 18 and n.sub.2 is the index of
refraction of air, the light propagating in glass substrate 18 and
incident at the glass--air interface will be totally internally
reflected.
However, when the propagating light is incident on a textured
surface at least a portion of the incident light is scattered, as
shown by arrows 20 (FIG. 1). At least some of the scattered light
can be directed out of the glass substrate. If the glass substrate
is to be used as a light source (e.g. a back light element) for an
LCD display device, glass substrate 18 can be positioned such that
the emitted light is directed through an adjacent LCD panel 12. The
textured surface may be the first major surface 22 farthest from
display panel 12 or the textured surface may be second major
surface 24 closest to display panel 12. In some instances, both
first and second surfaces may be textured. The optical
characteristics of glass substrate 18 are controllable inter alia,
via the process for producing the scattering elements 34. FIG. 4
depicts modeled data showing normalized power as a function of
scattering angle for a surface, e.g. first major surface 22,
comprising an RMS roughness (R.sub.q) of approximately 50
nanometers (nm). Curve 50 represents a surface wherein a
correlation length of the scattering elements is 200 nanometers,
while curve 52 represents a correlation length of 150 nm and curve
54 represents a correlation length of 100 nanometers. The data show
a trend toward increased forward scattering (reduced scattering
angle) as the correlation length is reduced. Textured surfaces
produced according to the present disclosure can provide a glass
substrate that does not create a visibly frosted appearance to the
glass. Such a frosted appearance reduces transparency and increases
haze. The surfaces resulting from the treatments described herein
have been shown to produce Distinctness of Image (DOI) values equal
to or greater than 90%.
It has been found that the sizes of the plurality of light
scattering elements 34 will affect the light scattering properties
of light emitter 14. For example, the textured first major surface
22 including light scattering elements 34 can comprise a root mean
square (RMS) roughness R(.sub.q) in a range from about 5 nanometers
to about 75 nanometers, for example in a range from about 20
nanometers to about 60 nanometers, and in other examples in a range
from about 20 nanometers to about 35 nanometers, or from about 20
nanometers to about 30 nanometers. In other examples, a root mean
square (RMS) roughness R(.sub.q) in a range from about 5 nanometers
to about 20 nanometers, or in a range from about 5 nanometers to
about 15 nanometers can be attained. A correlation length T of the
scattering elements can be equal to or less than about 252 nm, for
example equal to or less than 200 nm, equal to or less than 150 nm,
or equal to or less than 100 nm. As noted previously, it has been
found that a correlation length equal to or less than about 100 nm
is particularly advantageous when considered in the context of a
viewing angle goodness parameter, described in more detail herein.
The correlation length may be found, for example, by measuring a
roughness of the treated surface via a scanning probe microscopy
technique such as atomic force microscopy (AFM), and using a
software computational package, like Gwyddion.
Light source 16 may be one or a plurality of LEDs, or other
suitable light generating elements. Moreover, while FIG. 2
illustrates light source 16 along only a single edge or edge
surface of glass substrate 18, a light source 16 may be disposed
along one or more edge surfaces or one or more borders of glass
substrate 18. In one or more embodiments, the one or more edge
surfaces of glass substrate 18 may be beveled, as shown in FIG. 5,
and may include a metalized reflective surface. The bevel angle
.gamma. is chosen to redirect any light propagating within the
glass substrate 18 in one or more directions that reduce the escape
of light out of the at least one edge surface. Light emitter 14 may
optionally include a light redirecting (or blocking) border 40 to
improve the optical appearance near the edges of the glass
substrate 18.
One or more alternative embodiments may employ one or more light
sources 16 and associated structures of the type(s) disclosed in
PCT Publication No. WO12/058084 (PCT/US11/57032), published on May
3, 2012, the entire disclosure of which is incorporated herein in
its entirety by reference.
FIG. 6 illustrates a light emitter 14 wherein light source 16
comprises at least one light diffusing fiber 16a extending along
first edge 26 of glass substrate 18. The light diffusing fiber 16a
may be on the order of about 250-300 microns in diameter. Light
extracted from light diffusing fiber 16a, such as by scattering,
can be coupled into substrate 18.
In another embodiment shown in FIG. 7, one or more laser sources
may produce white (or semi-white) light to couple into light
diffusing fiber 16a and thereafter into substrate 18. In one or
more alternative embodiments, one or more laser sources, such as a
red laser source (RED), a green laser source (GREEN) and a blue
laser source (BLUE), may couple light energy of differing
wavelengths into a single light diffusing fiber 16a, or multiple
light diffusing fibers, in a way that couples such light into glass
substrate 18 and causes diffusion and scattering as discussed
previously. Using multiple laser sources permits the production of
any number of colors by adjusting the power level of each laser
source.
Further details concerning various structures and methodologies
associated with modulating the laser sources (e.g., time sequential
modulation) to achieve desirable color image functionality and
other details concerning the use of the light diffusing fiber 16a
may be found in: U.S. Patent Publication No. US2012/0275745A1,
published on Nov. 1, 2012; U.S. Patent Publication No.
US2013/0272014A1, published on Oct. 17, 2013; and; U.S. Patent
Publication No. US2014/0092623A1, published on Apr. 3, 2014, the
entire disclosures of which are incorporated herein by
reference.
Reference is now made to FIG. 8, a flow chart illustrating example
process steps in connection with producing light emitter 14, and
variants thereof, in accordance with embodiments disclosed
herein.
At step 100, glass substrate 18 may, if necessary, be processed to
make the glass substrate ready for the subsequent etching step. In
particular, glass substrate 18 may be ground and polished, if
necessary, to achieve a desirable thickness and/or flatness and/or
otherwise sized, and thereafter cleaned and washed in step 102.
Step 100 may include edge dressing of substrate 18, for example
edge/edge surface beveling. Washing may be accomplished, for
example, by exposing first major surface 22, alternatively second
major surface 24, or both first and second major surfaces 22, 24,
to a suitable detergent cleaning solution such as Semiclean KG,
with or without ultrasonic agitation, and multiple de-ionized
rinsing steps, followed by drying of the glass substrate. For
example, the glass substrate may be placed in a bath comprising the
cleaning solution, then rinsed by dipping or spraying with water.
The glass substrate should be cleaned sufficiently such that a
water contact angle of the surface to be etched is equal to or less
than about 20 degrees.
At step 104, if a major surface of the glass substrate 18 will not
be etched, for example second major surface 24, the major surface
not to be etched may optionally be laminated (preferably over the
entire surface) with a protective film 42 that resists or prevents
etching (see FIG. 9). It should be apparent that should it be
desired to etch both major surfaces of the glass substrate with the
etchant, a protective laminating film may not be required. In
another optional next step 106, the major surface or surfaces to be
etched, for example first major surface 22, may be subjected to a
process for removing contamination, such as adsorbed organic
contamination. Such process may include exposing at least first
major surface 22 to a plasma cleaning process as is known in the
art.
At step 108, the major surface of glass substrate 18 to be etched
is subjected to etching by exposing the surface to an etchant
comprising an organic solvent, for example acetic acid (AA), and an
inorganic acid, such as ammonium fluoride (NH.sub.4F), for a time
and at a temperature sufficient to obtain the requisite roughness
and correlations lengths. The etchant may comprise an organic
solvent, for example acetic acid, in a range from about 10% by
weight to about 98% by weight. In further examples, the organic
solvent may be in a range from about 10% to 80% by weight, in a
range from about 20% to about 70% by weight, or in a range from
about 30% to about 60% by weight. In still other examples the
organic solvent may be in a range from about 80% by weight to about
98% by weight, for example in a range from about 90% by weight to
about 98% by weight or in a range from about 92% by weight to about
98% by weight.
The etchant may further comprise an inorganic acid in a range from
about 0.5% by weight to about 50% by weight. For example, the
etchant may comprise an inorganic acid in a range from about 10% by
weight to about 40% by weight, from about 15% by weight to about
30% by weight. In other examples the etchant may comprise an
inorganic acid in an amount from about 0.5% to about 10% by weight,
or from about 0.5% by weight to about 6% by weight. In certain
embodiments the etchant may contain water (e.g. de-ionized water)
in an amount equal to or less than 8% by weight, for example in a
range from about 0.5% by weight to about 6.0% by weight, from about
0.5% by weight to about 4.0% by weight, or from about 0.5% by
weight to about 2.0% by weight. Optimal concentrations may vary
depending on glass composition and environmental conditions, such
as dew point, which can affect physisorbed water on the glass
surface, and the resultant texture desired.
In some embodiments certain other additives may be incorporated
into the etchant. For example, a dye may be added to the etchant to
add color and produce a visual aid for rinsing. In addition, as
previously described, viscosity modification components may be
added to increase the viscosity of the etchant and enable contact
by slot, slide or curtain coating etchant on glass vs. dipping, to
provide a light emitter with uniform appearance. Alternatively,
artistic effects can be achieved for when non-uniform light
extraction is required by applying etchant using spraying or
painting techniques. A high viscosity etchant may reduce the vapor
pressure and thereby reduce vapor-induced defects. The viscosity of
the etchant can be adjusted to be compatible with the selected
method of application as needed. Suitable polymers such as
polycaprolactone that are soluble in acetic acid may be used to
modify the rheology of the etch solutions.
The etch time may range from about 30 seconds to about 10 minutes,
although other times may be employed if such other times yield
desirable results. For example, exposure times in a range from
about 1 minute to about 4 minutes have been found useful for many
glass compositions. The temperature of the etchant may be in a
range from about 18.degree. C. to about 90.degree. C., for example
in a range from about 18.degree. C. to about 40.degree. C., in a
range from about 18.degree. C. to about 35.degree. C., in a range
from about 18.degree. C. to about 30.degree. C., in a range from
about 18.degree. C. to about 25.degree. C., or even in a range from
about 18.degree. C. to about 22.degree. C. Etchant temperatures in
the lower ranges, for example, ranges in the 18.degree. C. to
30.degree. C. range, are favored since this can reduce vapor
pressure and produces fewer vapor-related defects on the glass.
Again, the etch time and etchant temperature will have an effect on
the feature size, shapes, and distribution of the resulting light
scattering elements 34, which are nanometer-scale protrusions
extending from the etched surface of the glass substrate and
produced by the etching process. Etch texture can vary with glass
composition. Accordingly, etchant recipes optimized for one glass
composition may require modification to remain effective for other
compositions. Such modification is typically accomplished through
experimentation.
In some embodiments, as best seen in FIG. 9, if a surface is not to
be etched, e.g. second major surface 24, that surface may be
protected by applying an etchant-resistant protective film 42 to
the surface. Etchant-resistant protective film 42 may be removed
after the etching step(s). Table 1 below presents non-limiting
examples of some suitable etchant solutions and etch times.
TABLE-US-00001 TABLE 1 Weight % etch components Etch Solution
Ammonium Time, number Acetic Acid Fluoride Water min 1 92.8 5.4 1.8
2.5 2 94.1 4.1 1.8 3.5 3 95.8 1.4 2.9 4.1 4 93.4 3.8 2.8 5.1 5 94.9
1.1 4.0 5.5 6 92.1 2.2 5.8 5.5 8 91.0 8.0 1.0 9.0 9 91.0 1.0 8.0
9.0 10 91.0 4.5 4.5 7.0
Thus, the etchant should include an organic solvent, e.g. acetic
acid, in a concentration from about 90.0 percent by weight to about
96 percent by weight, and ammonium fluoride in a concentration in a
range from about 1.0 percent by weight to about 8.0 percent by
weight. The etchant may include water in an amount equal to or less
than about 8.0 percent by weight, for example from about 1.0
percent by weight to about 6.0 percent by weight. In addition, it
has been found that the temperature of the glass substrate itself
at the time the glass substrate is exposed to the etchant can in
some instances also affect etching results. Accordingly, the glass
substrate when exposed to the etchant may be at a temperature in a
range from about 20.degree. C. to about 60.degree. C., for example
in a range from about 20.degree. C. to about 50.degree. C., or in a
range from about 30.degree. C. to about 40.degree. C. (see Example
3). The optimal temperature will depend on glass type,
environmental conditions and the desired texture. Etch times may
extend from about 1 minute to about 10 minutes, although other etch
times as may be needed to achieve the desired surface texture may
also be used. As will be discussed further below, the foregoing
chemistry may be adjusted to accommodate different applications and
different glass compositions.
At step 110, glass substrate 18 can be rinsed with a rinsing
solution, for example water, in one or more rinsing (de-acidifying)
actions. Alternatively, in some embodiments the glass substrate can
be soaked in 1M H.sub.2SO.sub.4 for up to 1 minute to remove
precipitated crystal residue on the surface of the glass substrate.
However, other mineral acids may be substituted for
H.sub.2SO.sub.4, for example HCl or HNO.sub.3. The rinsing solution
may be heated in one or more of the rinsing actions. In some
embodiments the rinsing solution may include other fluids in which
precipitant from the etching process is dissolvable.
At an optional step 112, the glass substrate may be subject to an
ion exchange process. If such a process is desired, then the
process can begin with preparing at step 100 a glass substrate 18
capable of being ion exchanged. For example, ion-exchangeable
glasses suitable for use in embodiments described herein include
without limitation alkali aluminosilicate glasses or alkali
aluminoborosilicate glasses, though other glass compositions may be
substituted. As used herein, being capable of ion exchange means a
glass capable of exchanging cations located at or near the surface
of the glass substrate 18 with cations of the same valence that are
either larger or smaller in size.
The ion exchange process is carried out by immersion of the glass
substrate 18 into a molten salt bath for a predetermined period of
time, where ions within the glass substrate at or near the surface
thereof are exchanged for larger metal ions, for example, from the
salt bath. By way of example, the molten salt bath may include
potassium nitrate (KNO.sub.3), the temperature of the molten salt
bath may be within a range from about 400.degree. C. to about
500.degree. C., and the predetermined time period may be within a
range from about 4 hours to 24 hours, for example in a range from
about 4 hours to 10 hours. Incorporation of larger ions into glass
substrate 18 strengthens surfaces of the glass substrate by
creating a compressive stress in a near-surface region. A
corresponding tensile stress is induced within a central region of
the glass substrate 18 to balance the compressive stress.
By way of further example, sodium ions within glass substrate 18
may be replaced by potassium ions from the molten salt bath, though
other alkali metal ions having a larger atomic radius, such as
rubidium or cesium, may replace smaller alkali metal ions in the
glass. According to particular embodiments, smaller alkali metal
ions in glass substrate 18 may be replaced by Ag+ ions. Similarly,
other alkali metal salts such as, but not limited to, sulfates,
halides, and the like may be used in the ion exchange process. The
replacement of smaller ions by larger ions at a temperature below
that at which the glass network can relax produces a distribution
of ions across the surface of the glass substrate 18 that results
in a stress profile. The larger volume of the incoming ion produces
a compressive stress (CS) on the surface and tension (central
tension, or CT) in the center region of the glass substrate 18. At
optional step 114 the glass substrate after ion exchange is
subjected to a final water rinse followed by drying.
It should be apparent from FIG. 1, for example, that the intensity
of the scattered light as a function of propagation distance will
be dependent at least on the number of "bounces" as the light
propagates generally along axis 38. Thus, light that is scattered
nearer light source 16 will typically have a greater intensity than
light that is scattered farther from light source 16. Moreover,
intrinsic attenuation within the light guide will reduce the
intensity of the propagating light. Accordingly, methods to control
the scattering to account for these optical losses can be applied.
In one example, a deposition of micrometer-sized features after the
etching can be used to control the light scattering as a function
of distance across a surface of the glass substrate. This
deposition process can be implemented, for example, by ink-jet
printing, silk screening or other suitable deposition process.
As best shown in FIGS. 1, 2 and FIG. 6, etched first major surface
22 can be provided with a pattern of deposited material comprising
discrete layers 44 at step 116, hereinafter "dots" 44. The dots 44
suppress scattering at first major surface 22. As seen in FIG. 2,
while locally randomized, dots 44 are arranged on a macro scale in
a graduated configuration such that the spatial density of dots,
defined by a percent coverage of a unit surface area by the dot
material, decreases as one moves away from light source 16 along
axis 46. For example, a spatial density of dots proximate first
edge 26 can be at a percent coverage of 75%, whereas a spatial
density of dots proximate second edge 28 can be at a percent
coverage of 25%. It should be understood, however, that the spatial
density of dots can be configured in different percentages based on
the desired effect. In other examples the spatial density can vary
in a range from about 90% to about 10%, in a range from about 60%
to about 40%. The variation in spatial density of the dots can vary
linearly or nonlinearly as necessary to match the light attenuation
between the maximum spatial density and the minimum spatial density
or the desired optical effect. In comparison, the spatial density
along a line parallel with first edge 26 can be substantially
uniform, so that no substantial gradient exists along axis 43. The
plurality of dots 44 may be configured to be non-overlapping. To
avoid being visually apparent to an observer, the plurality of dots
44 may be transparent, and should have an index of refraction that
is within .+-.10% of the index of refraction of the underlying
substrate. The closer the index matching, the more effective the
dots will be at suppressing scattering. Accordingly, in some
embodiments the index of refraction of the dots can be within
.+-.5% of the index of refraction of the underlying substrate. The
spatial density of the dots can be varied depending upon how many
edges of the substrate are lighted by coupling light from a
suitable light source. For example, using the example described
supra, if the glass substrate is lighted from both first edge 26
and second edge 28, the density of dots may be high at both first
edge 26 and second edge 28, since the intensity of the light is
greatest proximate the light source, and decreasing from each edge
as one moves toward the middle of the glass substrate, midway
between the two edges (assuming, for example, the light intensity
at each edge is the same). A similar rationale applies if all four
edges are lighted such that the lowest suppression of scattering
occurs within the central portion of the light guide, yet a
substantially uniform amount of light is extracted across the
surface of the glass substrate to produce substantially uniform
illumination.
Each dot 44 may be formed by depositing a predetermined amount of a
suitable material, e.g. a polymer resin, having an index of
refraction approximately equal to the index of refraction of the
glass substrate to which they are applied (e.g., within 10%), and
at a prescribed position on the etched surface of glass substrate
18. Any number of suitable resins may be employed. For example, one
suitable resin is Accuglass.RTM. T-11 produced by the Honeywell
Corporation. Accuglass.RTM. T-11 is a family of heat-cured
methylsiloxane polymer resins useful for planarization of media,
particular in the field of integrated circuits. Accuglass.RTM. T-11
includes an index of refraction of 1.39 @ 633 nm. It should be
noted, however, that dots 44 need not be a polymer resin. For
example, dots 44 could be a transparent conductive oxide material
having a similar index of refraction or any other material that
includes an index of refraction closely matching the index of
refraction of the underlying substrate and is simultaneously
suitably transparent. Deposition of dots 44 may be performed by any
suitable deposition technique compatible with the nature of the
material deposited. For example, polymer resin can be deposited by
an ink jet printer, or by silk screen printing. For example, a
FujiFilm.RTM. Dimatix.TM. 2831 ink jet printer with a 1 picoLiter
print head has been successfully employed for this purpose. In the
case of ink jet deposition, a desired dot configuration can be
assembled as a digital bit map and provided to the printer. Each
dot may comprise a diameter in a range from about 15 micrometers to
about 30 micrometers, for example from about 20 micrometers to
about 28 micrometers or from about 22 micrometers to about 26
micrometers. A maximum thickness t of each dot may be in a range
from about 4 nm to about 10 nm, for example, from about 4 nm to
about 8 nm. If the dots are deposited via an ink jet printer, the
volume of polymer resin can be in a range from about 1 picoLiters
(pL) to about 15 pL. Ink jet printing has so far been shown to be
the most controllable and consistent method of depositing the dots.
Moreover, the ability to easily program the deposition pattern, and
if necessary make changes thereto, is particularly advantageous.
After the deposition process is completed, the deposited resin may
then be cured at an elevated temperature to harden the resin. If
the resin is instead curable by an ultraviolet light, the resin may
be cured by exposure to the requisite wavelength of light. It
should be noted that the volume of ink used during the deposition
of the dots is controlled such that the ink does not protrude
significantly above the surface of the substrate they have been
deposited on.
As described supra, an area of the etched surface covered by a dot
44 extracts less light through scattering than a non-covered area
by enhancing total internal reflection (conversely, by suppressing
scattering). Thus, the deposition of dots on a textured surface
provides a means for controlling the brightness at specific
locations on the textured surface by controlling the percentage of
surface area covered by dots 44 as a function of position on the
textured surface. Once the dots have been deposited, protective
film 42 applied to the un-etched surface of the glass substrate may
be removed if desired.
In still another aspect, production of substrates comprising a
textured surface can be produced in a production-friendly manner
that largely eliminates messy wet chemical etching of individual
substrates.
In a first step, a master mold can be produced in accordance with
steps described previously, for example through wet chemical
etching to obtain a substrate with a surface texture, the textured
surface having an RMS surface roughness equal to or less than 75
nm, for example in a range from about 5 nm to about 75 nanometers
and wherein a correlation length of the surface texture is equal to
or less than 150 nanometers, for example in a range from greater
than zero to equal to or less than 150 nanometers, or in a range
from greater than zero to equal to or less than 100 nanometers.
In a second step, an uncured elastomer can be applied to the
textured surface of the master mold to coat the textured surface.
Once the textured surface has been coated with the uncured
elastomer, the elastomer can be cured in accordance with the curing
method of the particular elastomer. For example, if the elastomer
is an ultraviolet light (UV) curable resin, the coated textured
surface can be exposed to a suitable UV light from a UV light
source to cure the elastomer. In other embodiments, the uncured
elastomer may be a thermoplastic material that is cured with
exposure to heat. When curing is finished, the cured elastomer
coating can be removed from the textured surface of the master mold
to produce an elastomer production mold in a third step.
To produce additional substrates including a textured surface
comprising a surface roughness and correlation length identical to
the surface texture of the master mold, one or more substrates, for
example glass substrates, can be coated with a suitable UV-curable
resin on at least one major surface of the one or more substrates.
The UV-curable resin should be selected such that upon curing, the
UV-curable resin attains an index of refraction that matches or
nearly matches the index of refraction of the substrate (e.g.
within .+-.10% of the index of refraction of the substrate). The
production mold produced in the foregoing process may then be
pressed into the UV-curable resin, after which the UV-curable resin
is cured by exposing the UV-curable resin to UV light in accordance
with the curing requirements of the particular resin. Once the
UV-curable resin has been cured, the elastomer production mold can
be removed, leaving a substrate with at least one textured major
surface having an RMS surface roughness equal to or less than 75
nanometers, for example in a range from about 5 nanometers to about
75 nanometers and wherein a correlation length of the surface
texture is equal to or less than 150 nanometers, for example in a
range from greater than zero to equal to or less than 150
nanometers, or in a range from greater than zero to equal to or
less than 100 nanometers.
In some embodiments the substrate may be a glass substrate. A
thickness of the glass substrate can be, for example, equal to or
less than 3 millimeters, for example, in a range from about 0.01
millimeters to about 3 millimeters, in a range from about 0.01
millimeters to about 2 millimeters, in a range from about 0.01
millimeters to about 1 millimeters, in a range from about 0.01
millimeters to about 0.07 millimeters. In some embodiments the
glass substrate can be a flexible glass substrate having a
thickness in a range from about 0.01 millimeters to about 0.3
millimeters, in a range from about 0.01 millimeters to about 0.2
millimeters, or in a range from about 0.01 millimeters to about 0.1
millimeters. The flexible glass substrate may be a flexible glass
ribbon. In some embodiments the flexible glass ribbon can be in the
form of a roll, wherein the UV-curable resin is applied to at least
one major surface of the flexible glass ribbon as or after the
flexible glass ribbon is dispensed from the roll.
FIG. 10 illustrates a process wherein at least one major surface of
a flexible glass ribbon can be processed to produce at least one
textured major surface in a continuous manner. As shown in FIG. 10,
a flexible glass ribbon 120 is configured in a roll 122 at a
pay-out station 124. The flexible glass ribbon comprises two
longitudinal edges extending along a length of the flexible glass
ribbon, the two longitudinal edges defining the lateral width of
two major surfaces 126, 128 of the flexible glass ribbon. The
flexible glass ribbon 120 is payed out from roll 122 in a
conveyance direction 130 along a predetermined conveyance path and
coating 132 of a suitable index-matched UV-curable resin is applied
to the least one major surface of the flexible glass ribbon, for
example major surface 126. The at least one major surface of the
flexible glass ribbon may then be contacted with an embossing drum
134 including a textured surface 136 and, simultaneous with the
contacting, the index-matched resin is cured by exposing the resin
to a UV light from UV light sources 138. The embossing drum surface
contacting the UV-curable resin can include, for example, a
production mold manufactured in accordance with the foregoing
process, wherein the production mold is attached to a peripheral
surface of embossing drum 134. As the embossing drum rotates and
the flexible glass ribbon continues advancing along the
predetermined conveyance path, the production mold ceases contact
with the cured UV-curable resin and comes into contact with uncured
UV-curable resin in a continuous manner. The upstream (leading)
portion of the flexible glass ribbon, including a polymer surface
layer comprising a textured surface, may then be taken up by
receiving roll 140 at take up station 142.
In another embodiment, embossing drum 134 can have the appropriate
texture formed directly into a peripheral surface thereof. For
example, a laser can be used to produce a texture directly on the
outer circumferential peripheral surface of the embossing, such as
by laser etching (engraving) or ablation. The laser may heat up the
surface and subsequently vaporize the surface of the embossing
drum, or may ablate the surface. In some examples a pulsed,
short-duration laser beam can be used to produce pitting of the
surface in a pattern programmed into the laser controller, the
pattern conforming to a predetermined surface roughness such as the
surface roughness and correlation length described herein.
Polymer films produced in a manner as described above, where a
texture is applied to a surface thereof, can be used in a variety
of applications. For example, in one such application, a
transparent glass substrate including a polymer film comprising a
texture can be used in a projection viewing system. In such as
system, an image screen is produced comprising a glass substrate
including textured surface as previously described. The textured
surface may be formed on a first surface of the glass substrate
itself, such as by etching, or the textured surface may be formed
in a polymer layer that was deposited on a surface of the first
glass substrate. The glass screen may then be mounted, for example
positioned in a frame or stand. A projector can then project an
image onto the glass screen. The light forming the image is
scattered by the texture present on the textured surface such that
the image is visible on the glass screen. In some embodiments a
second glass substrate may be joined to the first glass substrate
such that the textured surface is sandwiched between the first
glass substrate and the second glass substrate. The glass screen
may include a surface roughness equal to or less than 100
nanometers, for example an RMS surface roughness (R.sub.q) in a
range from about 5 nanometers to 75 nanometers, and a correlation
length equal to or less than 150 nanometers, such as greater than 0
nanometers but equal to or less than 150 nanometers. In some
examples the correlation length is greater than 0 nanometers but
equal to or less than 100 nanometers.
FIG. 12 illustrates a projection system 200 comprising projection
screen 202 including a textured substrate 204. Projection screen
202 is shown as a free-standing structure supported in an upright
posture by a base 207. The textured substrate can be a glass
substrate, a polymer substrate or a combination of polymer and
glass as discussed more fully below. An image 208 may be projected
onto projection screen 202 from projecting device 210. A cover
glass 218 may be positioned adjacent textured substrate 204.
In one embodiment, shown in FIG. 13A, projection screen 202
comprises textured glass substrate 204 including a surface texture
as described supra. Textured glass substrate 204 may include, for
example, a surface 214 etched with an etchant as previously
described to produce the requisite surface roughness (and
correlation length if applicable) on at least one surface of the
glass substrate. An opposing surface, here shown as surface 216,
may or may not be etched to produce a desired texture. A cover
substrate 218 may be bonded to the textured surface 214 of glass
substrate 204 if desired in order to protect textured surface 206
from damage.
In another embodiment, shown in FIG. 13B, projection screen 202 may
comprise a glass layer, for example untextured glass substrate 205,
and a polymer layer 212 disposed on the glass layer. Polymer layer
212 includes a surface texture having the surface roughness
characteristics described supra. For example, the surface texture
can be produced via a micro or nano-replication technique as
described above, or any other suitable replication process. The
surface texture can be applied to the polymer layer after
deposition of the polymer layer to the glass substrate, or the
polymer layer may be added to the glass substrate, for example as a
pre-textured film. As shown in FIG. 12 and FIG. 13C, a cover glass
substrate 218 may be positioned over the polymer layer 212. Cover
glass 218 may be configured with touch functionality.
Alternatively, non-textured surface 216 may be provided with touch
functionality.
In another aspect, a textured glass substrate configured to
minimize electrostatic charging of the glass substrate, such as
through triboelectrification that can occur during contact
separation, is disclosed.
Flat panel display glass used to build a display panel, and
particularly that portion of the display panel including thin film
transistors consists of two sides, a functional side ("backplane")
upon which thin film transistors (TFTs) may be built (A-side) and a
non-functional B-side. During processing, the B-side glass contacts
a variety of materials (i.e. paper, metals, plastics, rubbers,
ceramics, etc.,) and can accumulate an electrostatic charge through
triboelectrification. For example, when the glass substrate is
introduced into the production line and an interleafing material,
for example a laminated paper, is peeled from the glass substrate,
the glass substrate can accumulate an electrostatic charge.
Moreover, during the manufacturing process for semiconductor
deposition, the glass substrate is commonly placed on a chucking
table, B-side in contact with the chucking table, where the
deposition is performed. The chucking table may, for example,
restrain the glass via one or more vacuum ports in the chucking
table during processing. When the glass substrate is removed from
the chucking table, the B-side of the glass substrate can be
electrostatically charged through triboelectrification and/or
contact electrification. Such electrostatic charge can cause many
problems. For example, the glass substrate can be adhered to the
chucking table by the electrostatic charge and the glass substrate
subsequently broken into pieces when an attempt is made to remove
the glass substrate from the chucking table. Moreover, due to the
electrostatic charge, particles and dust can be attracted to the
glass surface and contaminate it. More importantly, the release of
electrostatic charge from the B-side to the A-side (so-called
electrostatic discharge, ESD) can cause failure of the TFT gate
and/or line damage on the A-side that reduces product yield.
Methods described herein may be used to minutely texture a glass
surface by reducing he contact area in a manner that effectively
reduces the intimacy of contact during triboelectrification and/or
contact electrification, resulting in reduced glass voltage or
surface charge, without a noticeable reduction in the transparency
of the glass, for example with minimal haze. As previously
described, an organic solvent is introduced to an inorganic acid to
produce rapid localized precipitation that forms crystal features
on a glass substrate surface. These precipitates are the etching
byproducts, normally fluorosilicate salts, which mask the
underlying glass surface and hinder etching in these locations.
Residual crystalline precipitates are dissolved away during a
subsequent hot water wash or with an acid wash, leaving texture
features on the glass surface as a result of the etching. By
adjusting the organic solvent-to-etchant ratio or etch time or
etchant temperature a wide range of texture roughness can be
obtained, from nanometer to micrometer range.
Methods described herein can be applied to almost all of the
traditional etchants used in the glass industry. For example, the
inorganic acid could be hydrogen fluoride acid, buffered hydrogen
fluoride acid (or the mixture of ammonium fluoride and hydrogen
fluoride), sodium fluoride and their mixtures with mineral acids
including hydrogen chloride, sulfuric acid, phosphoric acid and
nitric acid. The organic solvent must be miscible with the etchant.
Suitable organic solvents include without limitation acetic acid,
polyethylene glycol, isopropanol alcohol, ethanol, methanol, and
others.
Accordingly, in some embodiments the processes described herein may
be altered as necessary. For example, if the intent is to reduce
electrostatic charging and not to produce a light guide, such
aspects as viewing angle may not be important considerations.
Additionally, changes in glass type and/or composition may further
warrant adjustments to the chemistry involved in the etching
process.
In one example process, a glass substrate is initially washed with
a KOH solution to remove organic contaminants and dust on the
surface as a pristine glass surface is needed to achieve uniformly
distributed protrusions (texture features) on the glass surface.
Other washing solutions may be substituted as needed. The presence
of contaminants or dust on the surface can act as nucleation seeds.
These nucleation seeds can induce crystallization around them,
resulting in a non-uniform surface texture.
Next, the glass substrate is soaked in a solution containing an
organic solvent in a range from about 10% by weight to about 90% by
weight, for example in a range from about 10% to about 80% by
weight, in a range from about 20% to about 70% by weight, in a
range from about 20% by weight to about 30% by weight or in a range
from about 40% by weight to about 60% by weight. The etchant may
further include an inorganic acid in a range from about 1% to about
50% by weight, for example in a range from about 5% to about 40% by
weight, or in a range from about 10% by weight to about 30% by
weight. For example, in some embodiments a solution containing
NH.sub.4F in a range from about 1% to 20% by weight and 5% to about
50% by weight acetic acid or polyethylene glycol can be selected.
Depending on the desired texture roughness, the etching time can
range from about 30 seconds to a few minutes, for example in a
range from about 30 seconds to about 4 minutes, from about 30
seconds to about 3 minutes or from about 30 seconds to about 2
minutes. The solution may be static in some examples, while the
solution can be stirred or otherwise agitated in other examples.
For example, the etchant may be recirculated. Experiments have
shown no discernible difference in surface texture based on etchant
agitation within the etchant concentrations and etch times
described.
After etching is completed, the glass substrate can be soaked in 1M
H.sub.2SO.sub.4 for up to 1 minute to remove crystal residues on
the surface. However, the H.sub.2SO.sub.4 acid wash can be replaced
by other mineral acids such as HCl or HNO.sub.3. Alternatively, or
in addition, a hot water wash may be applied. A low pH value (or
high temperature) can increase the solubility of the precipitated
crystals. Consequently, a pH greater than 7, for example greater
than 8, is desired. After the wash the glass substrate may be
rinsed with water to remove the acid residue, then dried.
The resulting average roughness found to be effective to reduce
surface voltage (e.g. electrostatic charging), expressed as an
average roughness (R.sub.a) is typically in a range from about 0.4
nanometers to about 10 nanometers. Typical haze values are equal to
or less than 1% but may in some examples extend to as great as 6%.
Accordingly, specific solution recipes as described above can be
varied within the disclosed ranges as necessary to attain a surface
within the foregoing constraints.
Using the treatment methods described herein can result in a
reduction of surface voltage exhibited by a glass substrate surface
from about 20% to about 70%, for example in a range from about 23%
to about 67%, over the untreated substrate surface when tested via
the disclosed peel test. In the context of reducing the danger from
electrostatic discharge and the potential damage to sensitive
electronic components, such a change in acquired electrostatic
charge is significant and useful.
However, further disclosure reveals a contact-area-based metric
that can be used to characterize the advantageous component of
surface topography most impactful to electrostatic charging
behavior, and which describes regimes of ESD-advantaged surface
topography not captured or predicted by simple parameters like
average roughness (R.sub.a).
It is known that surface topography can be quantitatively defined
or described using a variety of metrics over a variety of length
scales with corresponding advantages and disadvantages to the
different data collection and quantification methods. Techniques
like atomic-force microscopy (AFM) or optical profilometry
described elsewhere herein provide images with height (z) for each
pixel measured in x and y dimensions. The size of the x-y pixel
determines the ability of the technique to resolve small features,
while the x-y field-of-view limits the ability to capture larger
features (or changes in topography that occur over wider scales).
An important note in this regard is that the essential character of
surface texture can be well-described using some metrics, but
totally fail by other metrics. For example, if one had a surface
texture covered by a regular array of 1 millimeter diameter dots of
100 nanometer height, but collect images 50 nanometer.times.50
nanometer in size and quantify the pattern using only correlation
length, one could completely fail to capture the salient aspects of
the topography. The degree to which this might be true depends on
the behavior one wishes to understand.
With this mind, average roughness (R.sub.a) or root mean square
roughness (R.sub.q) are some of the most commonly used parameters
to describe surface topography, and yield information about the
amplitude variation of heights on a surface (consistent with the
traditional sense of "roughness"). In practice, however, this
definition is only really appropriate when the topography being
described has more-or-less "random" variations of heights on its
surface. Ra and Rq fail to capture or suitably quantify the
presence of "features" on a surface that one can visibly see in an
image, instead serving to average them out over the field-of-view.
It is thus not the most appropriate metric for quantifying the
benefit of surface texture in terms of contact separation when the
surface contains topography other than random roughness.
Other parameters like skewness or kurtosis also seek to quantify
the asymmetry of the surface height profile about a mean surface
level. For example, as described previously, negative skew
indicates a predominance of "valleys," while positive skew is seen
on surfaces with a predominance of "peaks." Kurtosis measured the
deviation from "random" amplitude variations, with 3 representing a
normal/Gaussian distribution of heights. However, these single
values also do not suitably describe the height distributions of
nanometer-scale features on a surface considered important for
contact separation, and tend to underestimate the contribution of
surface features, for example, when the features are sparsely
distributed on a surface.
Accordingly, a metric is used that can define ESD-advantaged
surface topographies in a way that captures their essential benefit
through enhanced contact separation. Hereafter we define this
metric as a figure-of-merit (FOM) specific to contact separation,
per the following equation:
.delta..times..times..DELTA. ##EQU00004## with:
.DELTA..delta..ltoreq..delta.> ##EQU00005## where the parameter
h.sub.t is a "threshold height", effectively defining the
contribution of uppermost heights on a surface that comes into
contact with a second surface during a contact event. For example,
h.sub.t=0.95 corresponds to the top 5% of height values in an image
represents pixels that are "in contact" while h.sub.t=0.50
considers the top 50% of height values in an image represents
pixels that are "in contact."
An alternative way of thinking about this parameter is the "depth"
to which surface features elastically compress during a contact
event, such that multiple pixels at the peak of a feature are
rendered in contact, and thus not artificially restricted to the
single topmost pixel at the top of each peak.
The parameter .delta. is a "stand-off", representing a minimum
distance between the contacted area and an imaginary plane during a
contact event, .DELTA. effectively describes for pixels not
"in-contact" the extent of separation from the imaginary contact
plane during a contact event, CA is the contact area, given by the
number of pixels for which h.sub.i.gtoreq.h.sub.t, m is the
exponent of the denominator and weights the importance of
separation distance for pixels not "in-contact." In evaluations of
FOM, m is selected to be either 2 ("FOM2") or 6 ("FOM6"),
respectively representing physics of Coulombic (.about.r.sup.2) or
Lennard-Jones (.about.r.sup.6) interaction-length dependencies. The
surface height data for individual pixels is h.sub.i, where there
are N total surface height elements in the h.sub.i data.
In plain language, the first term in equation (5) captures the area
of the surface in "direct" contact with a secondary surface, in
this case mathematically equivalent to an imaginary plane, during a
contact event. The second term in the equation describes for
regions of the surface not in "direct" contact how separated it is
from the contacting surface. In the limit of improved ESD behavior,
FOM approaches zero as (a) effective surface-to-surface
contact-area is reduced (CA.fwdarw.0) and (b) those regions not in
direct contact are as far away from the second surface as possible
(.DELTA..fwdarw..infin.). Accordingly, in some examples a
substrate, such as a glass substrate, includes at least one
textured surface comprising an FOM in a range from 0 to 0.8, for
example in a range from greater than zero to about 0.78, in a range
from about 0.01 to about 0.7, in a range from about 0.05 to about
0.6, or in a range from about 0.01 to about 0.5 for m in a range
from 2 to 6 and wherein 0.1.ltoreq..delta..ltoreq.10 nm, for
example 0.1.ltoreq..delta..ltoreq.2 nm, and
0.5.ltoreq.h.sub.t.ltoreq.0.999, for example
0.75.ltoreq.h.sub.t.ltoreq.0.99.
It should be noted that the foregoing discussion in terms of
electrostatic charging does not by itself address the optical
qualities of the substrate should those optical properties be of
interest. That is, FOM is directed toward minimizing electrostatic
charging of a substrate based on, for example, contact separation,
and is equally applicable for transparent or opaque substrates
(e.g. glass substrates). Accordingly, for instances where attention
must be paid to a transparency of the substrate, the ESD
characteristics must be balanced with the optical qualities. To
wit, a substrate having optimal surface properties for
electrostatic charging may still present less than optimal optical
properties. Thus, obtaining a substrate comprising both an
acceptable FOM and acceptable transparency may require compromise
between the two requirements., and while it has been shown that FOM
is generally insensitive to roughness (either R.sub.a or R.sub.q),
these attributes may still have importance to transparency. In
further examples, a substrate comprising an FOM in a range from 0
to 0.8, for example in a range from greater than zero to about
0.78, in a range from about 0.01 to about 0.7, in a range from
about 0.05 to about 0.6, or in a range from about 0.01 to about 0.5
may also include an RMS roughness R.sub.q where 5
nanometers.ltoreq.Rq.ltoreq.75 nanometers, and wherein the texture
of the textured major surface includes a correlation length T where
0 nanometers<T.ltoreq.150 nanometers.
EXAMPLES
The present disclosure will be further clarified by the following
examples.
Example 1
Corning.RTM. Code 2320 glass samples were etched in solutions 1-6
from Table 1. Brightness and haze were measured and the data are
presented below in Table 2. Code 2320 glass is a sodium alumino
silicate glass.
TABLE-US-00002 TABLE 2 Decay exponent, Brightness @ 3 mm, Sample
Solution mm.sup.-1, b Watts % Haze 1 1 0.012 2.96E-07 0.791111 2 2
0.009 2.57E-07 0.346667 3 3 0.012 3.03E-07 0.715556 4 4 0.013
4.09E-07 0.798889 5 5 0.012 4.75E-07 0.856667 6 6 0.021 6.39E-07
4.261111
As the data show, samples exposed to etchants 1-5 are similar in
respect of the attenuation of light. A good metric to compare
brightness uniformity of the two conditions is b, the exponent in
an exponential fit (e.g., where the fit equation is of the form
y=Ae.sup.bx) of the brightness data. The data of Table 2 describe a
brightness decay exponent b ranging from about 0.009 mm.sup.-1 to
about 0.013 mm.sup.-1 and with etchant 2 exhibiting the lowest
brightness decay. The sample treated with etchant 6, although
initially brighter than the others, exhibited a steeper decay curve
with .lamda.=0.21 mm.sup.-1 Samples made with solutions 1-5 have
less than 1% haze, indicating a very high transparency to light in
the visible wavelength range and minimal scatter. Etchant 6 had the
highest haze (4.26%), but was still useable in a transparent
backlight element.
The luminance in candelas per meter square as a function of viewing
angle in degrees of the glass samples etched with solutions 1-6
above (curves 60, 62, 64, 66, 68, 70 respectively), for viewing
angles from -80 to 80 degrees, was measured and is depicted in the
plot of FIG. 14. This plot shows that luminance is most uniform for
the glass sample etched with etchant 2 (curve 62), and intermediate
uniformity resulted from using etchants 1, 3, 4 and 5. Etchants 1
and 3 produced almost identical results. Non-uniformity of
luminance increased significantly for the sample treated using
etchant 6.
Additionally, roughness parameters, including correlation lengths
of the samples treated with etchants 1, 2 and 6 through 9 were
measured and are presented below in Table 3, along with RMS
roughness R.sub.q. The columns, from left to right, are the sample
identification (7 through 17), the particular etch conditions used
(including glass temperature in degrees centigrade and etchant
solution identification according to Table 1), roughness parameters
as determined with an atomic force microscope (AFM) including
R.sub.a in nanometers, R.sub.q in nanometers, skew (R.sub.sk),
kurtosis (R.sub.ku), Gaussian correlation length (T) and a "viewing
angle goodness" (VAG) parameter. R.sub.a is the arithmetic mean
roughness value, R.sub.q is the quadratic mean roughness value (RMS
roughness), skew is a measure of the asymmetry of a probability
distribution. If skew <0, it can be a surface with valleys and
if R.sub.sk>0 it can be a flat surface with peaks. Values
numerically greater than 1.0 may indicate extreme valleys or peaks
on the surface. Kurtosis is a measure of the randomness of heights,
and of the sharpness of a surface. A perfectly random surface has a
value of 3; the farther the result is from 3, the less random and
more repetitive the surface is. Surfaces with spikes exhibit higher
kurtosis values; bumpy surfaces exhibit lower kurtosis values.
Since most people watch a display (e.g. television) from a viewing
angle in a range from about -30 degrees to about +30 degrees as
measured from a normal to the display panel surface, the luminance
(as measured in candela/meter.sup.2) should be high at those low
angles. However, displays are sometimes seen from higher angles, so
high luminance from about -80 degrees to about +80 degrees are also
highly desired. Accordingly, a display with uniform luminance at
angles of -80 to +80 degrees is said to have high viewing angle
uniformity. Displays lighted with a backlight element acquire
viewing angle uniformity primarily via the light scattering
properties of the backlight element.
When luminance is measured with equipment capable of measuring at
various viewing angles, the results can be displayed as a spherical
"radar" plot, or as an X/Y plot of viewing angle vs luminance at a
90 degree slice through the total hemispherical dataset, as shown
in FIG. 15 illustrating luminance as a function of viewing angle
L(.theta.) for 12 different backlights. Tools for acquiring viewing
angle luminance may be a Radiant Zemax imaging sphere and an ELDIM
EZ Contrast. Since comparing desirability of viewing angle plots
can be difficult, a metric called viewing angle goodness (VAG) was
developed. VAG is calculated by first multiplying the luminance
curve, L(.theta.), by the cosine of the viewing angle .theta. to
preferentially weight the light scattered at low viewing angles
.theta.. The average luminance at a viewing angle .theta. from 0 to
30 degrees is then divided by the average luminance at a viewing
angle .theta. from 30 to 80 degrees (see Equation 7 below).
.times..function..theta..times..times..times..theta..times..times..functi-
on..theta..times..times..times..theta..times. ##EQU00006##
VAG is a quantitative metric that accounts for the "goodness" of
the luminance as perceived by a viewer, for example a viewer of a
display lighted by a backlight element. The greater the VAG value,
the greater the perceived goodness (acceptability) of the
illumination.
The data from Table 3 show that generally the samples treated with
etchants #1 and #2 (samples 8 to 11 and 14 in particular) have
greater VAG values (e.g. greater than 1.5), and Gaussian
correlation lengths (T) substantially smaller than for the samples
treated with etchants 6 through 9. Generally, the smaller the
correlation length the better, with correlation lengths less than
100 nm being considered the most desired. In particular, sample 11
processed with solution 2 had the highest VAG value. A graph of the
relationship between correlation length and VAG is shown in FIG.
16.
TABLE-US-00003 TABLE 3 Etch Conditions Glass Temp, AFM Derived
Roughness Parameters Sample .degree. C. Solution Ra, nm Rq, nm Skew
Kurtosis T VAG 7 -19 2 48.4 62.2 -1.38 1.73 248.49 0.730499 8 37 2
25.6 33.4 -1.58 2.50 88.892 1.675984 9 60 2 16.1 21.8 -1.87 4.30
63.528 1.717703 10 22 1 23.6 31.6 -1.53 2.98 115.65 1.740163 11 22
2 19.9 26.7 -1.37 2.75 97.24 1.865436 12 22 6 36.3 43.9 -0.57 -0.14
187.58 0.905445 13 4 2 19.4 26.0 -1.56 3.19 131.59 1.402849 14 22 2
(with 19.4 26.0 -1.56 3.19 98.318 1.558392 out water) 15 22 7 19.2
25.7 -1.59 3.27 152.6 1.29123 16 22 9 19.3 25.9 -1.57 3.21 185.22
0.925579 17 22 8 19.3 25.9 -1.57 3.22 251.53 0.644086
To better understand the surface morphology of the etched glass,
porosity measurements were carried out on glass samples via argon
sorption measurements using a Micromeritics.RTM. ASAP.TM. 2420
accelerated surface area and porosimetry system. The samples were
all Corning code 2320 glass (150 mm.times.150 mm.times.0.7 mm)
manually dip-etched in acid solutions and rinsed in deionized
water. All samples were allowed to outgas for 24 hours prior to
measurement at 300.degree. C. All pore size measurements were based
on the volume of argon adsorbed via the Barrett-Joyner-Halenda
(BJH) model at a temperature of 87 K for pressures ranging from
4.72 to 734.12 millimeters Hg absolute pressure. All pore size
distribution fitting was conducted using a Rosin-Rammler model via
the Microsoft Excel Solver function to extract pore size
distributions into their respective modes.
For the purpose of this study the foregoing method was employed to
determine the volume fraction of the two types of pores that
appeared to present in the overall pore size distribution: pores
centered at an alpha (.alpha.) value above or below 1,000 Angstroms
(A) where alpha represents the 63rd percentile of a Weibull
distribution and corresponds generally to the peak value. The data
is shown in Table 4 below. For each sample, the parsed pore volume
fraction centered at 1,000 Angstroms is shown as the parameter
X.sub.1, while its counterpart population is calculated as
1-X.sub.1.
TABLE-US-00004 TABLE 4 Etch Glass Fraction Time, Temp., below
"Bimodality" Sample Solution min .degree. C. VAG 1000A [X.sub.1]
[X.sub.1(1 - X.sub.1)] 15 8 9 22 1.21 0.887 0.100231 16 10 9 22
1.06 0.869 0.113839 17 9 7 22 1.01 0.87 0.1131 18 2 3.5 -19 0.776
0.113 0.100231 19 2 3.5 37 1.616 0.795 0.162975 20 2 3.5 60 1.599
0.292 0.206736
After calculation of these fractions, a "bi-modality" parameter BP
was created by determining the product of these two volume
fractions [X.sub.1*(1-X.sub.1)]. The bi-modality was then
correlated with the VAG parameter using a second order polynomial
fitted to the data to determine any empirical correlations. This
fit, shown in FIG. 16, exhibits an empirical correlation with the
VAG parameter. The data suggest a unitless bimodality parameter
(BP) can adequately express a favorable surface porosity for
generating a scattering light guide with a high (.gtoreq.1.0) VAG
value. FIG. 17 suggests a BP value between about 0.16 and 0.22 as
being suitable for this purpose.
Example 2
In a series of experiments, 0.7 mm thick Corning code 2318 glass
samples having dimensions of 150 mm.times.150 mm were etched by
immersing the samples in a mixture of glacial acetic acid and
aqueous ammonium fluoride. Corning code 2318 is an alkali alumino
silicate glass. The samples were etched in a bath with a component
ratio of 15:6:4 of glacial acetic acid, water and ammonium
fluoride, respectively. The etchant bath immersion time was 3
minutes, after which the samples were rinsed in de-ionized water
and air dried. The samples were lighted along one edge surface
using an LED strip and the brightness of the samples' output at a
major surface thereof was measured starting from the edge nearest
the LED strip across to the opposite edge.
The etched surfaces of the samples were then printed with a
transparent resin in selected areas using a FujiFilm.RTM.
Dimatix.TM. 2831 ink jet printer with a 1 picoLiter print head,
depositing droplets of resin, each resin droplet containing
approximately 1 picoLiter of polymer resin. The polymer resin was
Accuglass.RTM. T-11. The gradient patterns used to print the
polymer resin were designed using Adobe.RTM. Illustrator software
and converted to a bit map format compatible with the printer
driver. The printed pattern was a (locally) random distribution of
polymer resin dots configured such that a coverage of dots per unit
area proximate one edge of each glass sample, i.e. the edge closed
to the LED strip, was greater than the coverage of dots per unit
area proximate the opposite edge. The gradient thus produced was an
approximately linear gradient from the first edge to the second
edge. The printing resulted in dots that were about 15-47 microns
in diameter. FIG. 18 is a photograph of a single dot deposited on
the roughed surface of the glass substrate, in a top-down view, and
shows the dot to be of an approximately circular shape with a
diameter of approximately 18 micrometers (.mu.m). FIG. 19 is a
second photograph of another dot (having a more oval or egg-like
shape). FIG. 20 is a plot of the profile of the dot of FIG. 19
showing the dot height on the left (y) axis and the width on the
bottom (x) axis along line 1 of FIG. 19.
The plot in FIG. 21 compares sample brightness after etching (both
without dots), curve 56 to the brightness of the same sample etched
and printed with the T-11 resin, curve 58. The sample brightness
was determined by measuring an optical power adjacent the surface
opposite the etched surface as a function of distance from the
light source. The brightness decay exponent b for the etched sample
before printing was 0.009 mm.sup.-1 and the brightness decay
exponent b for the etched and T-11 printed samples was 0.005
mm.sup.-1. Printing the T-11 resin on etched samples significantly
reduced the brightness decay exponent, leading to improved
illumination uniformity across the etched and printed sample.
The roughness of the etched but not printed region measured an RMS
roughness of 4 nanometers using a Zygo Newview.TM. model 7300 white
light interferometer with a 100.times. lens and a 2.times. zoom.
The dots resulting from the resin deposition increased the surface
RMS roughness (R.sub.q) to about 8 nm.
Example 3
In one laboratory experiment glass substrate samples were cooled in
a freezer to a temperature of about -19.degree. C.) and a
refrigerator to a temperature of about -4.degree. C., heated to
various temperatures in an oven, then immediately etched in a fresh
solution #2 (as shown in Table 1), and rinsed. Glass temperature
experiments were performed on two separate dates approximately 1
week apart, and strong positive log correlations (R.sup.2>0.95)
were exhibited between glass temperature prior to etch and the VAG
parameter, as seen in FIG. 22. However, this relationship was not
the same from one day to the next. It is believed that changes in
the relative humidity, room temperature and dew point between the
two dates may have been responsible for differences in the glass
temperature vs. VAG curves shown, and stabilizing the etching
environments could eliminate this variability.
Example 4
Samples of Corning code 2320 glass were etched with etchant
solution #2 from Table 1 for the times and at the temperatures
shown below in Table 5 below. Each sample was measured for haze and
transmittance using a BYK Haze-Gard Plus instrument from the Paul
N. Gardner Company, Inc. in accordance with ASTM D 1003, ASTM D
1044. The Haze-Gard Plus is capable of directly determining total
transmittance, haze and clarity. The instrument utilizes an
Illuminant C light source representing average day light with a
correlated color temperature of 6774 K. The results are provided
below in Table 5, and indicate very low haze (for example, less
than 2%) and high transmittance (greater than 94%, it being
understood that the maximum transmittance is bounded by 100%),
providing for a highly transparent glass substrate after treatment
with an etchant as described herein.
TABLE-US-00005 TABLE 5 Etch Etch Etch time temp % Sample solution
(minutes) .degree. C. % Haze Transmittance 21 #2 1 22 1.6 94 22 #2
1 22 0.86 94 23 #2 1 22 0.76 95 24 #2 1 22 0.71 95
Example 5
To understand the performance of an etch wax, 2.5 grams
polycaprolactone (Sigma a704105, MW 45k) were added to 50
milliliters acetic acid in a 100 milliliter round bottom glass
flask and stirred for 45 minutes in a hot water bath to dissolve
the polycaprolactone and produce a 5% polycaprolactone mixture. The
flask was then removed from the water bath and 0.9 milliliter
deionized water was gradually added while stirring. The mixture was
transferred to a Nalgene.TM. bottle, after which 2.1 grams of
ammonium fluoride was added and the mixture stirred for 1 hour.
Upon cooling, the mixture formed a 5% polycaprolactone etch
wax.
The etch wax was apply to a cleaned 150 millimeter.times.150
millimeter.times.2 millimeter sample of Corning.RTM. Code 4318
glass manually using a height adjustable Gardco Teflon coated
drawdown bar, 10.16 cm wide, set at <0.0254 cm gap. Code 4318 is
an alkali alumino silicate glass. The sample was laminated with a
polymer film to protect one major surface of the sample from the
etchant. The etch wax coating was left on the glass for 3.5
minutes, then rinsed in 3 water baths with agitation.
The glass sample was subsequently delaminated by removing the layer
of protective film covering the non-etched major surface of the
sample, and the sample was dried.
Additionally, both 10% and 15% polycaprolactone etch waxes were
also made using the same process. These required remixing using a
planetary mixer after initial stir since after several days of
sitting they became quite viscous (would not flow). This indicates
the solutions are thixotropic. After remixing, they were fine for
draw down application. The samples for all three applications were
then tested for VAG, percent haze and average luminance as
previously described. The data is presented in Table 6 below.
TABLE-US-00006 TABLE 6 Average Luminance, Sample % polycaprolactone
VAG % Haze Candelas/m.sup.2 25 5 1.74 0.67 Incomplete coating 26 5
1.74 0.67 477 27 10 1.73 1.14 652 28 15 1.87 0.41 Incomplete
coating 29 15 1.79 0.47 422
It has been found that the etching processes described herein are
capable of producing a surface texture that significantly reduces
electrostatic charging, and subsequent discharge, of glass
substrates, for example glass substrates used in the manufacture of
other display components, including display panels.
Example 6
In one example exploring the effect of a textured glass surface on
electrostatic charge characteristics of a glass substrate, glass
substrates were etched in a solution comprising ammonium fluoride
(NH.sub.4F) with a concentration of 1 weight percent and 5 weight
percent, and acetic acid in a concentration in a range from about
10 weight percent and 90 weight percent. The results describe the
impact of acid concentration and etch time on glass roughness using
a 2-factor fractal design developed using Minitab software. Table 7
lists the average glass roughness R.sub.a of a sample of
Corning.RTM. Lotus.TM. glass etched with the foregoing different
acid concentrations and etch times of 30 seconds and 120 seconds.
Lotus.TM. represents a family of substantially alkali-free alumino
silicate glasses having a high annealing point (for example,
greater than 765.degree. C.). A high annealing point can produce a
low rate of relaxation, and hence comparatively small amounts of
dimensional change, making the glass ideal for use as a backplane
substrate in low-temperature polysilicon processes. Glass surface
roughness was measured using a Zygo instrument over a scan area of
130 micrometers.times.180 micrometers. The data show that a broad
range of solution chemistry can be used to obtain very low surface
roughness.
TABLE-US-00007 TABLE 7 Acetic Acid NH.sub.4F Etch Time Roughness
Sample (vol. %) (wt. %) (sec) R.sub.a (nm) 30 10 1 30 0.369 31 10 1
120 0.439 32 10 5 30 0.472 33 10 5 120 0.361 34 90 1 30 0.717 35 90
1 120 9.287 36 90 5 30 7.270 37 90 5 120 6.662 Reference: 0.5M
NH.sub.4FHF, 0.610 90 sec., 40.degree. C., Lotus .TM.
Example 7
A solution composing of 20 weight percent of ammonium fluoride
(NH.sub.4F) and 50 weight percent of acetic acid was used to etch
Corning Lotus.TM. display glass samples. The glass was dipped in
the solution for the following periods of time: 30 seconds, 60
seconds, 90 seconds, 120 seconds and 180 seconds. Some glass
characteristics, such as average glass roughness (R.sub.a), the
maximum height of convex shapes on the glass surface (R.sub.z),
which is essentially a measure of the average distance between the
highest peak and lowest valley in each sampling length, glass haze
and glass clarity obtained at different etch time are summarized in
Table 8 below. Table 8 shows that slightly changing the etch time
using this solution dramatically increased both R.sub.a and
R.sub.z. As the etch time is extended to 120 seconds and 180
seconds, increased glass roughness values, 17.85 nanometers and
46.80 nanometers, respectively, were achieved. Visible haze was
observed on the glass surface at 180 seconds and the glass clarity
was not acceptable for some display applications. However, such a
surface roughness, even with haze, can be useful for other
potential applications such as an anti-glare surface or
anti-reflection surface, etc., and may also be used for backlight
application where transparency may not be required, such as for
standard opaque display back light elements. It can also be used
for general lighting purposes that do not require a high degree of
transparency.
TABLE-US-00008 TABLE 8 Etch Haze Voltage Sample time (s) R.sub.a
(nm) R.sub.z (nm) (%) Clarity Reduction (%) 38 30 0.79 71.47 0.14 5
23 39 60 2.86 155.33 0.33 5 24 40 90 6.98 291.33 0.94 4 28 41 120
17.85 324.00 3.70 2 53 42 180 46.80 386.67 16.30 1 67
The impact of etch time on surface topography was studied by atomic
force microscopy (AFM) with a scan size of 100 micrometer.times.100
micrometer. It was found that glass roughness increased
approximately exponentially as the etching time increased, and that
the roughness was tunable within the nanometer to micrometer range
through the methods disclosed herein. As the etching time is
extended, the precipitant crystals grow and more nucleation seeds
are formed. As a result, denser and larger crystal patterns were
observed on the AFM images depicted in FIGS. 23A-23C as the etching
time was increased from 30 seconds to 60 seconds and then 120
seconds. FIGS. 23A-23C illustrate the topography of Lotus.TM. glass
surfaces etched in 50% acetic acid and 20% ammonium fluoride for
30s, 60s and 120s over a mapping area of 100 micrometers.times.100
micrometers. Concurrently, glass roughness was found to have
increased from 0.79 nanometers to 19.5 nanometers over the range of
etch times. In comparison to traditional etching (the reference
solution without using organic solvents shown in Table 7), the
surface obtained by the in situ mask method described herein is
effective at reducing accumulated charge on glass surface for at
least two reasons. First, a large range of surface roughness can be
achieved whereas traditional etching tends to plateau at slightly
more than 1 nanometer, as shown in FIG. 24. FIG. 24 illustrates
average roughness R.sub.a as a function of etch depth for samples
of Corning.RTM. Eagle XG.RTM. glass and Corning.RTM. Lotus.TM.
glass and shows the plateauing of the depth of etch. R.sub.a in
FIG. 24 was measured by AFM with a scan size of 2
micrometers.times.2 micrometers. Second, the method creates
protrusions (peaks) on the glass surface, which enables a
relatively smaller contact surface area compared to the depressions
obtained by traditional etching, which is absent an organic
component in the solution. It should be noted that for the current
application, the achieved effects are constrained by both an
electrostatic consideration and optical considerations. The glass
must exhibit reduced electrostatic charging while also being highly
optically transparent with minimal haze. The small contact surface
area is preferred to more effectively reduce the accumulated
charges on glass substrate.
Example 8
To better understand the effect of surface texture on electrostatic
charging, Corning Lotus.TM. glass samples (180 mm.times.230
mm.times.0.5 mm) were etched in the foregoing NH.sub.4F/acetic acid
solution (50% acetic acid and 20% ammonium fluoride at 25.degree.
C.) for varying amounts of time as indicated in Table 7 and surface
voltage was measured with a commercially available lift tester
manufactured by the Harada Corporation using a grounded 304
stainless steel chuck plate. Reference samples were etched in a
0.2M NaF and 1M H.sub.3PO.sub.4 etchant solution. A -36 kPa vacuum
was created against the glass samples through a single vacuum port
in the chuck table and insulative Vespel pins (5 mm radius) were
used to lift the glass substrate samples from the chuck plate. The
lift pin speed was 10 mm/sec. Three samples per etch time were
sampled and run in random order. Six lift cycles per sample were
conducted with ionization used between lift cycles to neutralize
the samples. Values were reported at 80 mm pin height. The voltage
probe was configured to track with the glass substrate during lift
pin movement. The glass samples were cleaned using a wash with 4%
SemiClean KG, and the samples conditioned in a class 100 clean room
for 1 hour prior to test execution at a relative humidity of about
13%. The chuck and pins were HEPA vacuumed and wiped down with DI
cleanroom wipe 1 hour before testing. A sacrificial glass sample
was used to contact clean the chuck and pins at the start of
testing, side B then side A. Individual measurements were averaged
to produce the reported voltage change.
The percent reduction in voltage was calculated using the following
Equation (8): [(V.sub.o-V)/V.sub.o].times.100, (8) where V.sub.o is
the average voltage measured after contact separation of the glass
sheet before treatment and V is the average voltage measured after
contact separation of the glass sheet after treatment.
The glass voltage reduction in percent as a function of glass
roughness measured by AFM was plotted and is shown in FIG. 25. The
glass that was etched in the reference etchant (0.2M NaF/1M
H.sub.3PO.sub.4 at 40.degree. C. for 90 seconds) is shown as the
circle. The acetic acid/ammonium fluoride-etched samples are
represented by the square data points. A triangle denotes a sample
for which no surface treatment was conducted. The results indicate
that the glass voltage was significantly reduced in a range from
about 23% to about 67% as glass roughness varied from 0.79
nanometers to 54 nanometers when using the in situ mask etching
approach when compared to a traditional inorganic acid etchant.
Example 9
In manufacturing, glasses are traditionally wet etched using
various means such as dipping, spray, brush touched, etc.
Accordingly, another experiment was conducted to demonstrate that
the disclosed method is applicable to different agitation
conditions. As shown in FIGS. 26A and 26B, Lotus glass samples were
etched in a solution comprising 11% ammonium bifluoride
(NH.sub.4FHF) and 25% polyethylene glycol 20%, PG, (FIG. 26A) and a
solution composed of 20% ammonium fluoride (NH.sub.4F) and 50%
acetic acid, AA. The roughness was measured by a Zygo instrument
with a scan size of 180 micrometers.times.130 micrometers. Samples
having visible haze on the glass substrate after treatment are
circled. Glass samples were dipped in a static solution (symbol:
diamond), while other samples were dipped in a stirred solution
(symbol: square). The data indicate no significant difference in
respect of the resulting glass roughness.
For further comparison, the etchant was changed to ammonium
bifluoride, ABF, (NH.sub.4FHF) and polyethylene glycol (PG) as the
organic solvent component, which was again used to etch Corning
Lotus glass (FIG. 26B). Similarly, the experiment was conducted in
both static and stirring solutions. As expected, the same magnitude
of glass roughness is achieved in both conditions. This example
implies that the method is applicable to either static or dynamic
etching condition and is applicable to any combination of
fluorinated acid or salt and an organic solvent (i.e., acetic acid,
polyethylene glycol).
Example 10
As previously described, for display applications the electrostatic
charge-enhanced glass must perform satisfactorily both
electrostatically and optically. Generally, roughening a glass
surface may facilitate a reduction of accumulated electrostatic
charge on the glass substrate, but at the same time it may cause
the optical problems due to the scattering of light on a rough
surface. For display applications, the glass should typically be
transparent, although electrostatic charge may be considered even
for opaque substrates. The glass clarity was therefore evaluated by
visual inspection by lighting the glass samples from an edge
surface thereof, and a clarity value assigned. While this approach
is admittedly subjective, it was adequate to evaluate clarity on a
relative basis. Glass substrate samples were etched in a first
solution of 20% by weight NH.sub.4F and 50% by weight acetic acid.
Additional glass samples were etched in a second solution of 11% by
weight NH.sub.4FHF and 25% by weight polyethylene glycol. It was
observed that for a 90 second exposure to the etchant the obtained
average glass roughness (R.sub.a) was 6.98 and haze was 0.94% (see
Table 8), and the glass clarity was degraded. Haze was measured for
each sample and plotted as a function of glass roughness and is
shown in FIGS. 27A and 27B. FIG. 27A depicts haze as a function of
R.sub.a obtained through in situ mask etching approach with the
first solution of 20% by weight NH.sub.4F and 50% by weight acetic
acid. Glass surface roughness was measured by AFM with a scan size
of 100 micrometers.times.100 micrometers. FIG. 27B plots haze as a
function of R.sub.a for glass etched in the second solution of 11%
by weight NH.sub.4FHF and 25% by weight polyethylene glycol. Glass
surface roughness was measured by a Zygo instrument with a scan
size of 180 micrometers.times.130 micrometers. The dashed line
represents a haze value equals to 1%, considered to be an optimal
maximum haze for some applications. With the exception of several
outlier examples, the data show that glass surface haze can be
controlled within an acceptable range, e.g. equal to or less than
about 6%. The results further suggest that to maintain optimal
clarity while at the same time facilitating a reduction in
electrostatic charging, the glass roughness should be controlled in
a range from about 0.4 nanometer to about 10 nanometer, with haze
equal to or less than about 1%.
The glass surface composition for the samples was then analyzed by
scanning electron microscopy-electron dispersive X-ray spectrometry
(SEM-EDX). The composition of the surface protrusions (peaks) was
found to be identical to the glass substrate composition. There was
no etching by-product residue, (NH.sub.4).sub.2SiF.sub.6, found on
the glass surface, meaning the acid rinse after the etching step
efficiently removed the in situ mask material from the glass
surface. This result supports a conclusion that the addition of an
organic solvent to an etchant solution facilitates the nucleation
of crystals, the etching byproduct, on the glass. These crystals
mask the underlying glass surface and hinder etching in these
locations. Residual crystalline precipitates can then be dissolved
away during a subsequent acid wash or hot water wash, leaving
textured features on the glass surface.
Example 11
In another experiment, the performance of the FOM metric and its
correlation to ESD response was evaluated. Glass substrate samples
were etched with a variety of etchants. All samples were 180
millimeters.times.230 millimeters.times.0.5 millimeters of
Corning.RTM. Lotus.TM. glass. The glass substrates were then
subjected to a lift test and surface voltage was measured.
A first etchant comprised 1.5M HCl and 1.5 M HF produced in a 7
liter batch by combining 862 milliliters HCl, 381 milliliters HF
and 5757 milliliters deionized H.sub.2O. The samples were installed
together in a carriage and bathed in the first etchant at a
temperature of 30.degree. C. for 60 seconds, then rinsed in
deionized water at room temperature for 30 seconds.
A second etchant comprised 1M H3PO4 and 0.35M NaF produced in a 7
liter batch by combining 479 ml H.sub.3PO.sub.4, 103 grams NaF and
6521 ml deionized H.sub.2O. The samples were installed together in
a carriage and bathed in the second etchant at a temperature of
40.degree. C. for 81 seconds, then rinsed in deionized water at
room temperature for 30 seconds.
A third etchant comprised 20% by weight NH.sub.4F and 50% by volume
acetic acid produced in a 1 liter batch by combining 200 grams
NH.sub.4F, 500 ml acetic acid and 300 milliliters deionized
H.sub.2O. The samples were installed together in a carriage and
bathed in the third etchant at room temperature for 40 seconds,
then rinsed in 1 liter of an acid solution comprising produced by
combining 56.1 milliliters H.sub.2SO.sub.4 and 943.9 milliliters
deionized water at room temperature for 30 seconds. This acid rinse
was followed by further rinsing in deionized water.
A fourth etchant comprised 20% by weight NH.sub.4F and 50% by
volume acetic acid produced in a 1 liter batch by combining 200
grams NH.sub.4F, 500 ml acetic acid and 300 milliliters deionized
H.sub.2O. The samples were installed together in a carriage and
bathed in the fourth etchant at room temperature for 80 seconds,
then rinsed in 1 liter of an acid solution comprising produced by
combining 56.1 milliliters H.sub.2SO.sub.4 and 943.9 milliliters
deionized water at room temperature for 30 seconds. This acid rinse
was followed by further rinsing in deionized water.
A fifth etchant comprised 4.1% by weight NH.sub.4F and 94.1% by
weight acetic acid produced in a 1000 gram batch by combining 41
grams NH.sub.4F, 896 milliliters acetic acid and 18 milliliters
deionized H.sub.2O. The samples were installed together in a
carriage and bathed in the fifth etchant at room temperature for
210 seconds, then rinsed in deionized water at room temperature for
30 seconds.
Shown in FIG. 28A through 28F are AFM images at feature-appropriate
X/Y/Z scales the electrostatic charging results (FIG. 29) for the
different conditions highlighting the progression of reducing the
electrostatic-induced voltage as a function of surface preparation,
include topographies that represent "random" roughness, as well as
some containing obvious topographic features.
After etching the glass substrates were subjected to a lift test
with a commercially available lift tester manufactured by the
Harada Corporation using a grounded 304 stainless steel chuck
plate. Surface voltage was measured during the lift testing. A -36
kPa vacuum was created against the glass samples through a single
vacuum port in the chuck table and insulative Vespel pins (5 mm
radius) were used to lift the glass substrate samples from the
chuck plate. The lift pin speed was 10 mm/sec. Four samples per
etch time were sampled and run in random order. Six lift cycles per
sample were conducted with ionization used between lift cycles to
neutralize the samples. Values were reported at an 80 mm pin
height. The voltage probe was configured to track with the glass
substrate during lift pin movement. The glass samples were cleaned
using a wash with 4% SemiClean KG, and the samples conditioned in a
class 100 clean room for 1 hour prior to test execution at a
relative humidity of about 12%-13%. The chuck and pins were HEPA
vacuumed and wiped down with DI cleanroom wipe 1 hour before
testing. A sacrificial glass sample was used to contact clean the
chuck and pins at start of testing, side B then side A. Individual
measurements were averaged to produce the reported peak voltage
change.
Peak surface voltage in kilovolts for the samples is depicted in
FIG. 29, shown with 95% confidence limits. The samples are
referenced along the bottom (x-axis) as C0 through C5, where C0
refers to a control sample that did not undergo etching, and where
C1 through C5 refer to the first through fifth etchants. It should
be apparent from FIG. 29 that all five etchants had a noticeable
effect on electrostatic charging. However, it should also be noted
that the samples etched with an inorganic acid and an organic
solvent (ammonium fluoride and acetic acid, respectively), produced
significantly better results that etchants 1 through 3.
In addition to voltage testing, 50 millimeter.times.50 millimeter
samples of the etched glass were tested for haze and transmittance
using a Haze-Gard Plus instrument, and the data is presented in
Table 9 below. In this instance clarity was a measured quantity
obtained directly from the Haze-Gard instrument and not a
subjective grading as previously performed. The samples were also
measured for Ra, Rq correlation length T. The data were also used
to calculate FOM for m=2.
TABLE-US-00009 TABLE 9 Sam- R.sub.a R.sub.q T FOM Trans- ple (nm)
(nm) (nm) (2) Haze mittance Clarity C0 0.28 0.34 6.20 0.77 0.12
94.3 100 C1 0.68 0.85 18.77 0.61 0.10 94.3 100 C2 0.57 0.72 14.97
0.64 0.07 94.3 100 C3 1.02 3.93 177.76 0.70 0.12 94.3 100 C4 7.49
17.07 357.34 0.66 1.08 94.3 99.8 C5 42.4 54.92 283.40 0.22 17.3
94.3 99.8
As is apparent from Table 9, all the sample sets exhibited good
haze performance except for sample set C5. Good haze performance
from the control samples, C0, was expected, since the surfaces were
pristine and untreated. Sample sets C1 through C4 exhibit some
improvement in electrostatic charging performance, as represented
by a reduction in peak voltage when compared to the control sample
set C0. However, sample set C5, while showing the greatest
improvement in peak voltage reduction, also showed a high haze
value. Visual examination of the C5 samples revealed noticeable
haze. The high haze value for C5 compared to the results
attributable for earlier surface treatment with comparable
solutions used for light guide substrates can be attributed to the
differences in glass compositions, further reinforcing the need to
adapt the chemistry to account for differences in glass
composition.
For these data depicted in FIGS. 28A-F and 29, a series of plots
are shown in FIGS. 30A, 30B, 31A, 31B demonstrating the correlation
between the electrostatically-induced voltage and FOM for specific
values of m, h.sub.t ("thr"), and standoff .delta. ("offset").
Correlation coefficients (R.sup.2) are greater than 0.75 indicating
substantial correlative value to the metric.
To optimize the correlative value of a given topographical metric,
it is helpful to explore the impact of various mathematical
parameters going into its calculation, such as m, h.sub.t, and
.delta.. In the contour plots depicted in FIGS. 31A, 31B, the
variation in the correlation coefficient as a function of h.sub.t
and .delta. for two values of m (FOM2, FOM6) is highlighted. In the
plots, the x-axis "1-threshold" equates to (1-h.sub.t), and the
y-axis "offset" equates to .delta.. For these particular
topographies, the shaded regions represent a selection of h.sub.t
and .delta. values that tend to maximize the strength of the
correlation. The black dots indicate examples of particularly high
high correlation points.
The foregoing analysis shows that choosing the right h.sub.t is a
stronger factor than .delta. in having FOM correlate and predict an
electrostatic charging response. Typical values of h.sub.t might
range from 0.7 to 0.9, while .delta. ranges from 0.5 to 2.0, and
m=2 provides a higher R.sup.2 value than m=6, though the fit of
FOM2 to electrostatic charge data is somewhat eccentric. As with
any topographical metric, the selection of "best-fitting"
parameters like threshold heights may be influenced by specific
topographical feature dimensions, such that appropriate values can
be chosen for a given surface texture with some judgment. This
practice can even be performed when calculating R.sub.a on a
surface containing features, since an underlying parameter in the
calculation is the selection of a suitable image size (e.g., for
R.sub.a to be meaningful, one needs an image suitable to capture
the features of interest. That is, if one is trying to calculate
the roughness of a range of mountains, it is inappropriate to use
data zoomed in on one of the mountainsides.
The unique space wherein FOM and R.sub.a (R.sub.q) diverge can be
readily described using the idealized topographies shown in FIGS.
32A, 32B, 33A, 33B, 34A and 34B. FIGS. 32A-34B illustrate
alternately pairs of surface features in a surface plane (x-y
plane), and a corresponding slice in the vertical (height) plane
along the x-axis (height profile on the z-axis).
For a given R.sub.a being held constant, FOM can vary almost
independently from the limit of "pinholes" to "spikes" simply by
varying the lateral feature size. Likewise, at a given FOM (i.e.
contact-area), R.sub.a can vary almost independently by simply
increasing the depth of the pinholes (or the height of the spikes).
When short, narrow spikes are mixed with random roughness, it is
also likely to create a scenario where the contribution of the
spikes will hardly be detected in the calculation of R.sub.a.
However, in all situations a surface with a multitude of sharp
spikes providing contact separation will anticipate a much better
electrostatic charging response than a surface featuring pinholes
or random roughness alone.
This is illustrates graphically in FIGS. 35 and 36. FIG. 35 depicts
FOM (m=2) as the "duty cycle" of the idealistic features
(waveforms) depicted in FIGS. 32b, 33B and 34B vary (essentially,
as the peaks broaden). The top curve shows the progression of a
high FOM (m=2) as it varies from 1 toward a midpoint 0.5, whereas
the bottom curve shows the progression of a low FOM (m=2) as it
varies from zero toward a midpoint 0.5. The plot shows that for a
constant peak-to-valley height (roughness), as the peaks broaden
(and hence the contact area between the substrate and a contacting
surface increases), the FOM varies. On the other hand, FIG. 36
shows that for a broad range of roughness, for each the conditions
of FIG. 35 (varying duty cycle), the FOM (m=2), is constant,
illustrating that FOM is independent of roughness.
As a whole, the variety of FOM and R.sub.a (R.sub.q) responses are
depicted schematically in FIG. 37, showing a regime where FOM &
R.sub.a (R.sub.q) mostly correlate well, but also highlighting the
topographies where R.sub.a would fail to correlate with
electrostatic charging response.
For the embodiments described above, both R.sub.a (R.sub.q)
actually still correlate with electrostatic charging response, and
cross-correlate with FOM. This happens to be true for the surface
topographies on these particular embodiments. However, it is not
true as a rule, and there are clearly topographies for which FOM
and R.sub.a (R.sub.q) will diverge. In these instances, FOM will
serve as a better correlative descriptor for reduced contact
area.
It will be apparent to those skilled in the art that various
modifications and variations can be made to disclosed embodiments
without departing from the spirit and scope of the disclosure. Thus
it is intended that the present disclosure cover the modifications
and variations of these embodiments provided they come within the
scope of the appended claims and their equivalents.
* * * * *